migrations: strongly indicate dry runs

(cherry picked from commit 0696fcbe9b5cd66be6f6b3bb91d1e8e95fe6c21d)

migrations/nixos-mailserver-migration-03.py

@@ -113,6 +113,10 @@ def main(vmail_root: Path, layout: FolderLayout, dry_run: bool = True):
     for path in maybe_delete:
         print(f"# rm {str(path)}")
 
+    if dry_run:
+        print("\nNo changes were made.")
+        print("Run the script with `--execute` to apply the listed changes.")
+
 
 if __name__ == "__main__":
     parser = argparse.ArgumentParser(