Marks Linux_pixel_7pro hello_world_impeller to be flaky (flutter#173699)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_pixel_7pro hello_world_impeller"
}
-->
Issue link: flutter#173698

Author: fluttergithubbot

.ci.yaml

@@ -2462,6 +2462,7 @@ targets:
 
   # Uses Impeller.
   - name: Linux_pixel_7pro hello_world_impeller
+    bringup: true # Flaky https://github.com/flutter/flutter/issues/173698
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60