[monorepo] mark local engine builds as bringup. (#160627)

These tests are slightly flakey due to the macos cache issue. These
aren't actually tests, instead the prepopulate an RBE cache - so the
failure shouldn't close the tree, because cold caches isn't a failure
state (its just unfortunate).

engine/src/flutter/.ci.yaml

@@ -50,6 +50,7 @@ targets:
     enabled_branches:
       - master
     recipe: engine_v2/engine_v2
+    bringup: true
     properties:
       config_name: local_engine
     # local_engine schedules a bunch of other builds, so it's likely to timeout waiting for those builds to run.