feat(nixos): pin nixpkgs registry to nixpkgs flake input

modules/nixos/common.nix

@@ -1,4 +1,4 @@
-{ ... }:
+{ nixpkgs, ... }:
 
 {
   # Enable experimental features
@@ -29,6 +29,11 @@
     "hyprland.cachix.org-1:a7pgxzMz7+chwVL3/pzj6jIBMioiJM7ypFP8PwtkuGc="
   ];
 
+  # Pin the nix registry
+  nix.registry = {
+    nixpkgs.flake = nixpkgs;
+  };
+
   # Allow unfree packages
   nixpkgs.config.allowUnfree = true;
 }