Improves a few things:
- Use the free macos-14 runners for arm64 builds. They are not as fast
the macos-13-xlarge runners used before, but they are free for public
repositories, meaning we can start doing nightly arm64 builds.
- Use different nix install and cache actions which are faster, and uses
a single action cache key, which avoids cache rate limit errors
which was slowing down cache create/restore times.
- Generally refactor and tidy up various bits and pieces in workflows.
