refactor: add overlay stub

flake.nix

@@ -46,8 +46,18 @@
       ];
 
       perSystem =
-        { pkgs, inputs' , ... }:
+        { pkgs, inputs' , system, lib, ... }:
         {
+          _module.args.pkgs = import inputs.nixpkgs {
+            inherit system;
+            overlays = [
+              (final: prev: {
+                # overlay here
+              })
+            ];
+            config = { };
+          };
+
           devShells = rec {
             default = pkgs.mkShellNoCC { packages = ci.nativeBuildInputs; };