Builds using macOS 11 just takes a stupidly long time as, as Homebrew
seems to have dropped support, meaning all homebrew dependencies are
installed from source, rather than get installed from bottles.
Hence let's try upgrading minimum macOS version from 11 (Big Sur), to
12 (Monteray).
This should help reduce costs for Apple Silicon builds, as they are not
available on any form of free tier of GitHub Actions at the moment.
The build step that actually runs Emacs' configure script, and compiles
Emacs, is the only step that actually needs to run on the target
hardware architecture. The rest should be architecture agnostic.
This makes the workflow setup more flexible, and less copy/pasting
between workflows. And also makes the process of adding nightly builds
for non-master branches easier too.