feat(common): enable system wide diff in the activation output

This helps me to review what changes could be problematic in advance.

Signed-off-by: Raito Bezarius <masterancpp@gmail.com>

common/base-server.nix

@@ -60,4 +60,14 @@
 
   security.acme.acceptTerms = true;
   security.acme.defaults.email = "infra@forkos.org";
+
+  # Enable system diffs.
+  system.activationScripts.system-diff = {
+    supportsDryActivation = true; # safe: only outputs to stdout
+    text = ''
+      if [ -e /run/current-system ]; then
+        PATH=$PATH:${pkgs.nix}/bin ${pkgs.nvd}/bin/nvd diff /run/current-system $systemConfig
+      fi
+    '';
+  };
 }