Follow-up on config settings: Build sdist, then install via wheel, config settings no passed through

[nschloe]

After helping to complete #3090 (thanks!), one can now pass config_settings to the build backends via, e.g.,

[testenv:.pkg]
config_settings_build_sdist =
  A = b
config_settings_build_wheel =
  X = y

I noticed though that when building sdists, the subsequent installation step

py3: install_package> python -I -m pip install --force-reinstall --no-deps /path/to/.tox/.tmp/package/1/foobar-1.0.0.tar.gz

again employs wheel building without the config settings being passed. They would have to be passed to to pip as --config-settings.

Would it be sensible to introduce another variable here, or should config_settings_build_wheel be used?

[gaborbernat]

We don't want another variable. config_settings_build_wheel could be passed on as an env var here perhaps 🤔