Add flake checks

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

flake.nix

@@ -28,7 +28,16 @@
       {
         packages.${system} = import ./packages { inherit pkgs; };
         formatter.${system} = treefmt.config.build.wrapper;
-        checks.${system}.formatting = treefmt.config.build.check inputs.self;
+
+        checks.${system} =
+          let
+            packages = pkgs.lib.mapAttrs' (
+              n: pkgs.lib.nameValuePair "package-${n}"
+            ) inputs.self.packages.${system};
+
+            formatter.formatting = treefmt.config.build.check inputs.self;
+          in
+          packages // formatter;
       }
     );
 }