This serves as an alternative to Homebrew. It should be much more stable
and cause less headaches over time for automated builds.
There should be no change to the end user experience of using the build
script, as it should still work with and use Homebrew by default.
Additionally, Nix provides older Apple SDKs, allowing us to run against
macOS 11.x SDKs. This allows the resulting Emacs.app builds to be
compatible with macOS 11.x and later versions.
In testing, this seems to be the case on macOS 11.x (x86_64) and macOS
12.x (arm64).
It can easily lead to issues where certain packages are not compatible
due to an older version already being installed. Hence we instead prefer
to just update all deps to the latest version
