Adding Garnix CI

haskell-template already uses Github Actions for CI, but you may also use Garnix. Garnix is a Nix based hosted CI service that integrates well with GitHub. Compared to Github Actions, Garnix CI jobs run faster and finally you get a free Nix cache as a result (no need to manually push to cachix).

You may use the following garnix.yaml to enable both Linux and macOS builds:

    - "*.aarch64-darwin.*"
    - "*.x86_64-linux.*"
    - "checks.*.main-hls"