Merge branch 'macos-ci' into master

ci: Build and cache packages for macOS via GitHub Actions

After spending a massive amount of time making RetroArch build on macOS
(github:NixOS/nixpkgs#102230), I've decided to make sure my packages
have the right platforms on their `meta.platforms` and `meta.broken`.

Looks like builds.sr.ht has FreeBSD images.  Might try to build for that
later.