[CP] Roll .ci.yaml changes into the LUCI configuration only when the master branch is updated (flutter#164155)

This is a cherry pick of an infrastructure fix.  The "ci_yaml flutter roller" job updates the LUCI builder configuration to match the contents of ci.yaml.  It should only run on the master branch because the older version of ci.yaml in a release branch may not contain newer builders that are needed by master branch PRs.

See flutter#163897

Author: jason-simmons

.ci.yaml

@@ -545,6 +545,9 @@ targets:
     recipe: infra/ci_yaml
     presubmit: false
     timeout: 30
+    enabled_branches:
+      # Don't run this on release branches
+      - master
     properties:
       tags: >
         ["framework", "hostonly", "shard", "linux"]