Add upgrade-diff script.

nodes/hosts/common/scripts/upgrade-diff.nix

@@ -0,0 +1,9 @@
+{ lib, pkgs, ... }: {
+  system.activationScripts.postUserActivation = {
+    text = ''
+      ${pkgs.nvd}/bin/nvd --nix-bin-dir=${pkgs.nix}/bin diff /run/current-system "$systemConfig"
+    '';
+  } // lib.optionalAttrs pkgs.stdenv.isLinux {
+    supportsDryActivation = true;
+  };
+}