this ensures that os.path.expanduser and pathlib.Path.expanduser work by default inside a tox environment on non-Windows systems, including those without /etc/passwd.

fixes #2702.

this patch is analogous to #519.

Thanks for contribution

Please, make sure you address all the checklists (for details on how see
development documentation)!

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
  • i haven't done this because it is nontrivial to add tests that run on a system without /etc/passwd. the easiest way to get such a system is an OCI container, e.g.

    #!/usr/bin/env -S sh -c '< "$0" podman run --rm -i -v "$(dirname "$(realpath "$0")")":/tox-ro:ro nixery.dev/bash/coreutils/python311/git bash "$@"'
    
    set -o errexit -o errtrace -o nounset -o pipefail
    shopt -s inherit_errexit
    
    cd "$(mktemp -d)"
    python -m venv .venv &> /dev/null
    . .venv/bin/activate
    cp -r /tox-ro /tox
    
    set -o xtrace
    
    pip install -q /tox
    mkdir proj
    cd proj
    mkdir -p src/hello
    touch pyproject.toml src/hello/__init__.py
    printenv HOME
    cat /etc/passwd || true
    tox --colored yes

    but it's perhaps not worth the overhead to do that for this somewhat esoteric test.
• added news fragment in docs/changelog folder
• updated/extended the documentation
  • i haven't done this because the default pass_env on various platforms is not documented:

    tox/docs/config.rst

    Lines 279 to 281 in 079f839

    	.. conf::
    	:keys: pass_env, passenv
    	:default: <empty list>