Leverage Cachix and GitHub actions for binary caching

[lytedev]

It would be awesome to use Cachix for caching all our builds so folks don't ever actually have to build stuff themselves! Additionally, it looks like the Cachix cache has already been created and we just need to make use of it: https://app.cachix.org/cache/ssbm-nix

Is this your cache, @djanatyn? Or should we setup a different one?

[lytedev]

Hold up, looks like this is already done?

https://github.com/djanatyn/ssbm-nix/blob/master/.github/workflows/builds.yaml

So we just need to add the nix config hints to the flake?

  nixConfig = {
    extra-substituters = ["ssbm-nix.cachix.org"];
    extra-trusted-public-keys = ["ssbm-nix.cachix.org-1:YN104LKAWaKQIecOphkftXgXlYZVK/IRHM1UD7WAIew="];
  };