Mark flavors_test_macos unflaky (#160990)

This should have been marked unflaky when
flutter/flutter#156943 was closed.

Author: jmagman

.ci.yaml

@@ -3974,7 +3974,6 @@ targets:
       - DEPS
 
   - name: Mac flavors_test_macos
-    bringup: true # Flaky https://github.com/flutter/flutter/issues/156943
     presubmit: false
     recipe: devicelab/devicelab_drone
     timeout: 60