Add flake checks

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

flake.nix

@@ -28,7 +28,11 @@
       {
         packages.${system} = import ./packages { inherit pkgs; };
         formatter.${system} = treefmt.config.build.wrapper;
-        checks.${system}.formatting = treefmt.config.build.check inputs.self;
+
+        checks.${system} = with inputs.self; {
+          packages = packages.${system};
+          formatter = treefmt.config.build.check inputs.self;
+        };
       }
     );
 }