Improve building Homebrew*.pkg

- split jobs into build/test/deploy
- test package on both macOS Intel and Apple Silicon
- cleanup some argument handling
- use `HOMEBREW_MACOS_OLDEST_SUPPORTED` naming to be consistent with
  `brew.sh`
- note in `brew.sh` that `Distribution.xml` also needs updated (and do
  so)
- various other little bits of style cleanup