Revamp nix flake

[nixith]

I decided to go with crane instead, it apparently has better caching capabilities. The check capabilities are nice as well if you ever want to add some stuff to gh actions, for example. If you ever add another cargo subprjoct or some random directory stuff it might need some extra work but it should mostly be fine. Happy to explain, at least at a high level, whatever is going on.

Also given how I did the commits it might be a good idea to just squash this down during the pull request, at least I think you can.

Currently this PR is waiting on #214, it works if you remove the Build.rs script locally.