Marks Linux_pixel_7pro new_gallery_impeller__transition_perf to be flaky (#139644)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_pixel_7pro new_gallery_impeller__transition_perf"
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Issue link: https://github.com/flutter/flutter/issues/139643

.ci.yaml

@@ -2519,6 +2519,7 @@ targets:
 
   # Pixel 7 Pro, Impeller (Vulkan)
   - name: Linux_pixel_7pro new_gallery_impeller__transition_perf
+    bringup: true # Flaky https://github.com/flutter/flutter/issues/139643
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60