This will allow for simplifying the CI, since build option overrides can
be passed to build_root.py without text-file manipulations.

- Choose login shell for all MacOS steps.
- Use homebrew and python-venv to install dependencies
- Use github checkout action to get the code

Now that overrides can be passed as arguments to the configure script,
no further modifications of text files are required.

Homebrew doesn't natively provide it, so it seems easier to use a
builtin.