Describe the bug

When running multiple builds, the css config of the first build is reused. When running a build with a different config later on, it won't read from the provided config but will use the one that was encountered first.

Reproduction

I've created a small demo in https://github.com/luukdv/vite-builds. When you run npm i && node build, it will create two directories inside dist. Both are identical because they're built using the postcss config of the first encountered build.

• Expected: the CSS file in dist/test-1 contains .text-green-500
• Actual: the CSS file in dist/test-1 contains .text-red-500 (the same as dist/test-2, since this one was built first)

System Info

  System:
    OS: macOS 11.4
    CPU: (8) x64 Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
    Memory: 1.44 GB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 14.15.4 - ~/.nvm/versions/node/v14.15.4/bin/node
    npm: 6.14.10 - ~/.nvm/versions/node/v14.15.4/bin/npm
  Browsers:
    Chrome: 91.0.4472.114
  npmPackages:
    vite: ^2.3.8 => 2.3.8 

Used package manager: npm