Marks Linux_pixel_7pro flavors_test to be flaky (#156956)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_pixel_7pro flavors_test"
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-->
Issue link: flutter/flutter#156955

Author: fluttergithubbot

.ci.yaml

@@ -2708,6 +2708,7 @@ targets:
       task_name: fast_scroll_large_images__memory
 
   - name: Linux_pixel_7pro flavors_test
+    bringup: true # Flaky https://github.com/flutter/flutter/issues/156955
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60