python.pkgs.buildPythonPackage: use distutils-cfg to block downloads, fixes #25428

While we tell pip not to fetch (with the `--no-index` option),
`setuptools` can do so itself. In the past we used a `distutils.cfg`
with `allow-hosts = None` to prevent setuptools from fetching itself.
This was removed when we started building wheels in
2562f94de4e4fd2ddc677187fa2e2848L69.

The `dist-utils.cfg` code was still there, and adding it to
`buildInputs` is sufficient.

Tested with python.pkgs.passlib by removing the `checkInputs` / `nose`.