Marks Windows plugin_test to be flaky (flutter#174117)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Windows plugin_test"
}
-->
Issue link: flutter#174116

Author: fluttergithubbot

.ci.yaml

@@ -6168,6 +6168,7 @@ targets:
       - DEPS
 
   - name: Windows plugin_test
+    bringup: true # Flaky https://github.com/flutter/flutter/issues/174116
     recipe: devicelab/devicelab_drone
     timeout: 60
     properties: