Add an expression for building and caching important packages in CI

We want to `nix-build` important packages against every push to this repo, to ensure that:

* Things build correctly
* Newly built packages get cached for Linux and macos

We'll need to define which packages these are. Probably at least we'll want all the Ruby versions we want to support (all 2.x and 3.x), and the gemset.nix that's generated by the Rails Nix project. We could run a separate CI on that project, or do it all here. I feel like doing it here is better for now.

Another thing we can do is cache common gems to speed up initial deployments a bit. This would require some clever work to get things built, but that would only need to happen once per gem-version+platform.

Start by reviewing the repository's existing CI configuration and the use of nix-build. Define the important packages, including supported Ruby 2.x and 3.x versions and the Rails Nix gemset.nix, then decide whether gemset builds belong here or in the Rails Nix project. Done means builds run on every push and newly built packages are cached for Linux and macOS.

Written by the indexing model from the issue text.