Marks Mac_ios ios_debug_workflow to be unflaky (flutter#174104)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac_ios ios_debug_workflow"
}
-->
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_ios%20ios_debug_workflow%22).
This test can be marked as unflaky.

Author: fluttergithubbot

.ci.yaml

@@ -5438,7 +5438,6 @@ targets:
 
   - name: Mac_ios ios_debug_workflow
     recipe: devicelab/devicelab_drone
-    bringup: true
     presubmit: false
     timeout: 60
     properties: