[HACK] Apply panel overlay in the whole flake

This is to circumvent the fact that I cannot do `nixpkgs.overlay` in the
node in the NixOS test. Probably we can still apply it somewhat locally,
but I couldn't think of a good solution.

flake.nix

@@ -32,6 +32,7 @@
       perSystem =
         {
           pkgs,
+          system,
           lib,
           inputs',
           ...
@@ -39,6 +40,11 @@
         {
           formatter = pkgs.nixfmt-rfc-style;
 
+          _module.args.pkgs = import inputs.nixpkgs {
+            inherit system;
+            overlays = [ (import ./panel/nix/overlay.nix) ];
+          };
+
           pre-commit.settings.hooks =
             let
               ## Add a directory here if pre-commit hooks shouldn't apply to it.

panel/nix/configuration.nix

@@ -136,8 +136,6 @@ in
   };
 
   config = mkIf cfg.enable {
-    nixpkgs.overlays = [ (import ./overlay.nix) ];
-
     environment.systemPackages = [ manage-admin ];
 
     services = {