fix: install our dependencies outside the project directory

[henryiii]

Testing a suggestion from #1487.

[henryiii]

There are serious networking issues with GHA currently, so needs to be rebuilt later.

[henryiii]

@pypa/cibuildwheel-team, what do you think of this patch? This is a fix for a weird problem when you have custom build command options in setup.cfg that causes Python < 3.10 to break when running pip in the build directory - which breaks pip installs, virtual environment creation, and modern pyproject.toml based build isolation. By making sure pip is not run in the build directory, the failure no longer happens on pip install <cibuildwheel's stuff>, making it easier for the package author to work around and make it work (though all the other stuff is still broken, like adding a pyproject.toml).

I'm neutral on this - I'm not very interested in making sure cibuldwheel works when pip install, python -m venv, and adding a pyproject.toml are all broken, but this seems like a low-cost "fix" that will help make a workaround possible and will keep cibuildwheel out of the tracebacks. :)

[joerick]

I'm -1 on this, it adds complexity to our code without offering a functionality benefit. If we were to add it, I'd like to include a test also, but adding a test that ensures broken projects still make it through seems a bit perverse!