Enable diff for system changes

modules/misc/common/default.nix

@@ -149,6 +149,12 @@
       ];
   };
 
+  system.activationScripts.diff = {
+    supportsDryActivation = true;
+    text = ''
+      ${pkgs.nvd}/bin/nvd --nix-bin-dir=${pkgs.nix}/bin diff /run/current-system "$systemConfig"
+    '';
+  };
 
   # This value determines the NixOS release from which the default
   # settings for stateful data, like file locations and database versions