nixos/renovate: Add note about linting systemd scripts

nixos/tests/renovate.nix

···
           enable = true;
           settings.server.HTTP_PORT = 3000;
         };
+        # Uncomment the next line to lint service scripts (Note: breaks VM startup; see #373166)
+        #systemd.enableStrictShellChecks = true;
       };
 
     testScript = ''