Marks Mac_x64 native_ui_tests_macos to be unflaky (#142598)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac_x64 native_ui_tests_macos"
}
-->
The test has been passing for [50 consecutive runs](https://data.corp.google.com/sites/flutter_infra_metrics_datasite/flutter_check_test_flakiness_status_dashboard/?p=BUILDER_NAME:%22Mac_x64%20native_ui_tests_macos%22).
This test can be marked as unflaky.

.ci.yaml

@@ -4832,7 +4832,6 @@ targets:
 
   - name: Mac_x64 native_ui_tests_macos
     recipe: devicelab/devicelab_drone
-    bringup: true
     timeout: 60
     properties:
       dependencies: >-