Move mac cache builder to bringup. (#49843)

This won't have major impact because older caches still exist but it will prevent the build from closing the tree.

flutter/flutter#141688

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: godofredoc

.ci.yaml

@@ -118,6 +118,7 @@ targets:
     timeout: 60
 
   - name: Mac builder_cache
+    bringup: true
     enabled_branches:
       - main
     recipe: engine_v2/cache