Marks Mac flutter_tool_startup__macos to be unflaky (#112094)

diff --git a/.ci.yaml b/.ci.yaml
index 4c22458..031f7a5 100644
--- a/.ci.yaml
+++ b/.ci.yaml
@@ -4504,7 +4504,6 @@
       task_name: flutter_tool_startup__linux
 
   - name: Mac flutter_tool_startup__macos
-    bringup: true
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60