nixos/nh: allow independent nh clean service (#513757)

        [ ];

    assertions = [

      # Not strictly required but probably a good assertion to have

      {

        assertion = cfg.clean.enable -> cfg.enable;

        message = "programs.nh.clean.enable requires programs.nh.enable";

      }

      {

        assertion = (cfg.flake != null) -> !(lib.hasSuffix ".nix" cfg.flake);

        message = "nh.flake must be a directory, not a nix file";