nixos/version: Warn about using the default of system.stateVersion

nixos/modules/testing/test-instrumentation.nix

@@ -129,6 +129,9 @@ in
     # Make sure we use the Guest Agent from the QEMU package for testing
     # to reduce the closure size required for the tests.
     services.qemuGuest.package = pkgs.qemu_test.ga;
+
+    # Squelch warning about unset system.stateVersion
+    system.stateVersion = lib.mkDefault lib.trivial.release;
   };
 
 }