Mark the ci_yaml roller task so that it is not backfilled. (flutter#131966)

In order to avoid new tasks from not being scheduled we are not going to backfill the autoroller because it can sometimes play transactions out of order causing issues with tasks that are scheduled but not known.

*List which issues are fixed by this PR. You must list at least one issue.*
Part of flutter#127063

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

Author: ricardoamador

.ci.yaml

@@ -333,6 +333,7 @@ targets:
     properties:
       tags: >
         ["framework", "hostonly", "shard", "linux"]
+      backfill: "false"
     runIf:
       - .ci.yaml