nix: Re-enable and fix basic nix flake check for nix package and module (#3425)

Co-authored-by: Weston Keele <wekeele@proton.me>

flake.nix

@@ -145,6 +145,7 @@
             nixpkgs.overlays = [ self.overlays.${system}.default ];
           };
 
-          checks = { };
+          checks =
+            import ./nix/tests { inherit pkgs; flake = self; };
         });
 }