pipenv install <package> fails when PIP_EXTRA_INDEX_URL environment variable is set (from version 2021.11.23)

### Issue description

Pipenv 2021.11.23 fails when I try to install a package using pipenv install <package>, if the environment variable PIP_EXTRA_INDEX_URL is set. I get the same problem in both windows and the ubuntu-latest docker image.

The following works:
* "pipenv install" with the package listed in Pipfile
* unsetting PIP_EXTRA_INDEX_URL
* rolliong back to 2021.11.15

### Expected result

Package installed and added to Pipfile

### Actual result

Package not installed and not added to Pipfile. Error message:

´´´
Traceback (most recent call last):
  File "C:\Users\IND\AppData\Local\Programs\Python\Python39\lib\runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Users\IND\AppData\Local\Programs\Python\Python39\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "C:\Users\IND\tmpenv\Scripts\pipenv.exe\__main__.py", line 7, in <module>
  File "c:\users\ind\tmpenv\lib\site-packages\pipenv\vendor\click\core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "c:\users\ind\tmpenv\lib\site-packages\pipenv\vendor\click\core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "c:\users\ind\tmpenv\lib\site-packages\pipenv\vendor\click\core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "c:\users\ind\tmpenv\lib\site-packages\pipenv\vendor\click\core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "c:\users\ind\tmpenv\lib\site-packages\pipenv\vendor\click\core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "c:\users\ind\tmpenv\lib\site-packages\pipenv\vendor\click\decorators.py", line 84, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "c:\users\ind\tmpenv\lib\site-packages\pipenv\vendor\click\core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "c:\users\ind\tmpenv\lib\site-packages\pipenv\cli\command.py", line 194, in install
    do_install(
  File "c:\users\ind\tmpenv\lib\site-packages\pipenv\core.py", line 2114, in do_install
    index, verify_ssl=index.startswith("https:")
AttributeError: 'list' object has no attribute 'startswith'
´´´

### Steps to replicate 

```
> set PIP_EXTRA_INDEX_URL=<extra_index_url>
> python -m venv tmpenv --upgrade-deps
> tmpenv\Scripts\activate
> pip install pipenv
> pipenv install pandas
```

-------------------------------------------------------------------------------

<details><summary>$ pipenv --support</summary>

Pipenv version: `'2021.11.23'`

Pipenv location: `'c:\\users\\ind\\tmpenv\\lib\\site-packages\\pipenv'`

Python location: `'c:\\users\\ind\\tmpenv\\scripts\\python.exe'`

Python installations found:

  - `3.9.4`: `C:\Users\IND\AppData\Local\Programs\Python\Python39\python.exe`

PEP 508 Information:

```
{'implementation_name': 'cpython',
 'implementation_version': '3.9.4',
 'os_name': 'nt',
 'platform_machine': 'AMD64',
 'platform_python_implementation': 'CPython',
 'platform_release': '10',
 'platform_system': 'Windows',
 'platform_version': '10.0.19041',
 'python_full_version': '3.9.4',
 'python_version': '3.9',
 'sys_platform': 'win32'}
```

System environment variables:

  - `PIP_EXTRA_INDEX_URL`
  - `PROMPT`
  - `VIRTUAL_ENV`
  - `_OLD_VIRTUAL_PATH`
  - `_OLD_VIRTUAL_PROMPT`
  - `__PSLOCKDOWNPOLICY`
  - `ALLUSERSPROFILE`
  - `APPDATA`
  - `CHOCOLATEYINSTALL`
  - `COMMONPROGRAMFILES`
  - `COMMONPROGRAMFILES(X86)`
  - `COMMONPROGRAMW6432`
  - `COMPUTERNAME`
  - `COMSPEC`
  - `DRIVERDATA`
  - `HOMEDRIVE`
  - `HOMEPATH`
  - `HOMESHARE`
  - `LOCALAPPDATA`
  - `LOGONSERVER`
  - `NUMBER_OF_PROCESSORS`
  - `ONEDRIVE`
  - `OS`
  - `PATH`
  - `PATHEXT`
  - `PROCESSOR_ARCHITECTURE`
  - `PROCESSOR_IDENTIFIER`
  - `PROCESSOR_LEVEL`
  - `PROCESSOR_REVISION`
  - `PROGRAMDATA`
  - `PROGRAMFILES`
  - `PROGRAMFILES(X86)`
  - `PROGRAMW6432`
  - `PSMODULEPATH`
  - `PUBLIC`
  - `PYENV`
  - `SNOW_INVENTORY_HOME`
  - `SYSTEMDRIVE`
  - `SYSTEMROOT`
  - `TEMP`
  - `TMP`
  - `UATDATA`
  - `USERDNSDOMAIN`
  - `USERDOMAIN`
  - `USERDOMAIN_ROAMINGPROFILE`
  - `USERNAME`
  - `USERPROFILE`
  - `WINDIR`
  - `WSLENV`
  - `WT_PROFILE_ID`
  - `WT_SESSION`
  - `ZES_ENABLE_SYSMAN`
  - `PIP_SHIMS_BASE_MODULE`
  - `PIP_DISABLE_PIP_VERSION_CHECK`
  - `PYTHONDONTWRITEBYTECODE`
  - `PIP_PYTHON_PATH`
  - `PYTHONFINDER_IGNORE_UNSUPPORTED`

PipenvÔÇôspecific environment variables:


DebugÔÇôspecific environment variables:

  - `PATH`: `C:\Users\IND\tmpenv\Scripts;C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\wbin;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\WINDOWS\System32\WindowsPowerShell\v1.0\;C:\WINDOWS\System32\OpenSSH\;C:\Program Files\nodejs\;C:\ProgramData\chocolatey\bin;C:\Users\IND\ExeFiles;;C:\Program Files\Docker\Docker\resources\bin;C:\ProgramData\DockerDesktop\version-bin;C:\Users\IND\.poetry\bin;C:\Users\IND\AppData\Local\Programs\Python\Python39\Scripts\;C:\Users\IND\AppData\Local\Programs\Python\Python39\;C:\Users\IND\AppData\Local\Programs\Python\Launcher\;C:\Users\IND\AppData\Local\Microsoft\WindowsApps;C:\Users\IND\AppData\Local\Programs\Microsoft VS Code\bin;C:\Users\IND\AppData\Local\Programs\Git\cmd;C:\Users\IND\AppData\Roaming\npm;C:\Users\IND\ExeFiles\azcopy_windows_amd64_10.10.0\azcopy_windows_amd64_10.10.0;C:\Users\IND\dapr;C:\Users\IND\.dapr\bin;C:\Users\IND\.dapr\bin;C:\Users\IND\Miniconda3;C:\Users\IND\Miniconda3\Library\mingw-w64\bin;C:\Users\IND\Miniconda3\Library\usr\bin;C:\Users\IND\Miniconda3\Library\bin;C:\Users\IND\Miniconda3\Scripts`
  - `VIRTUAL_ENV`: `C:\Users\IND\tmpenv`


---------------------------

Contents of `Pipfile` ('C:\\Users\\IND\\Pipfile'):

```toml
[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[packages]

[dev-packages]

[requires]
python_version = "3.9"

```

</details>



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pipenv install <package> fails when PIP_EXTRA_INDEX_URL environment variable is set (from version 2021.11.23) #4881

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