No test covering `config_settings` argument to `build`

[pganssle]

I've been playing a bit with mutmut, and while it's a bit noisy, it did find this meaningful mutation here:

-     if not config_settings:
+     if config_settings:
        config_settings = {}

I can confirm that this is also not tested in the integration tests.

A less important mutant is that apparently also the default value for skip_dependencies is never tested in _build_in_current_env (which might be an argument for just not giving it a default value).

[FFY00]

I think the following would also have caught it.

[coverage:run]
branch = true

I will add this to my TODO, we should enable branch coverage and add the missing tests.