ci: remove cachix action

[rycee]

All the extra binary cache lookups and uploads to Cachix make the build quite a bit slower than necessary.

[terlar]

The uploads could be sped up with useDaemon: true as they then will be done async instead of at the end. But I guess if the cache lookup and cache download doesn't help much there is no need for it at all..

[rycee]

@terlar Thanks for the feedback. Yeah, I think our use-case (creating an enormous number of small files without pinning Nixpkgs) is not good for Cachix so it doesn't really make a difference in runtime, just adds unnecessary load on the Cachix servers.

I think we eventually would benefit from moving the CI out of GitHub actions into a system with direct and persistent access to a Nix store.

We also run a lot of unnecessary tests, ideally we would be able to tell which tests are affected by a given PR. For example, it is pretty unnecessary to run git module tests when the PR only changed something in the bat module.