Marks Mac flutter_packaging_test to be unflaky (#123238)

Co-authored-by: godofredoc <godofredoc@google.com>

Author: fluttergithubbot

.ci.yaml

@@ -2725,7 +2725,6 @@ targets:
   - name: Mac flutter_packaging_test
     recipe: packaging/packaging
     timeout: 60
-    bringup: true
     properties:
       task_name: flutter_packaging
       tags: >