- 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.

build_root.py could be run locally without access to openstack if no
uploads of build caches are attempted. Here, all openstack-related
imports are moved to the function that actually does the upload, so the
rest of the script can be run standalone.