Marks Windows build_tests_1_6 to be unflaky (#137673)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Windows build_tests_1_6"
}
-->
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:%22Windows%20build_tests_1_6%22).
This test can be marked as unflaky.

.ci.yaml

@@ -4551,7 +4551,6 @@ targets:
   - name: Windows build_tests_1_6
     recipe: flutter/flutter_drone
     timeout: 60
-    bringup: true
     properties:
       add_recipes_cq: "true"
       dependencies: >-