Rewrite flake.nix to use nci

[ehllie]

The current version of flake.nix uses the buildRustPackage function to create flake outputs. The downside of that, is that the hash of the cargo dependencies needs to be written manually, meaning each time Cargo.toml or Cargo.lock changes, the flake derivations break. This change uses nci which does not need manual setting of the output hash, and should therefore not break anymore.