Marks Linux build_tests_1_5 to be unflaky (flutter#154991)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux build_tests_1_5"
}
-->
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%20build_tests_1_5%22).
This test can be marked as unflaky.

Author: fluttergithubbot

.ci.yaml

@@ -446,7 +446,6 @@ targets:
       - .ci.yaml
 
   - name: Linux build_tests_1_5
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties: