Marks Linux_pixel_7pro hello_world_impeller to be unflaky (#135372)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_pixel_7pro hello_world_impeller"
}
-->
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:%22Linux_pixel_7pro%20hello_world_impeller%22).
This test can be marked as unflaky.

.ci.yaml

@@ -1780,7 +1780,6 @@ targets:
     recipe: devicelab/devicelab_drone
     presubmit: false
     # Uses Impeller.
-    bringup: true
     timeout: 60
     properties:
       tags: >