Run a Linux builder in the background

[lucperkins]

I recently added a module to my nix-darwin config (see here) that runs a Linux builder as a background process on my Apple M1. I was inspired by this comment on Discourse and the fantastic work of Gabriella Gonzalez. If you'd like, I'd be more than happy to add this capability to your configs here as an alternative to the Docker builder. I can now run nix build <flake>#packages.aarch64-linux.<package> without a second thought on macOS and it's been a real game changer for me.