This was discussed before, but no issue was created, so I do this.

Basically, we could add a nix cache (e.g. of a free one is attic: https://discourse.nixos.org/t/introducing-attic-a-self-hostable-nix-binary-cache-server/24343). Then the CI would push there and we could benefit by pulling from that cache, instead of building on our machine.

Maybe this is less urgent now, since GHC 9.6.6 is in cache already, it was annoying when we had to build things with flake (as that particular GHC version wasn't in the cache).

Anyways, if we start nix from scratch, it still builds quite a few things (haskell-related).