Marks Linux tool_integration_tests_5_7 to be unflaky (#165476)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux tool_integration_tests_5_7"
}
-->
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%20tool_integration_tests_5_7%22).
This test can be marked as unflaky.

Author: fluttergithubbot

.ci.yaml

@@ -1356,7 +1356,6 @@ targets:
       - DEPS
 
   - name: Linux tool_integration_tests_5_7
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties: