Reenable 'Linux packages_autoroller' in prod with kms key update (#135316)

The Github token was recreated and uploaded to GCS. I have verified a passing run with the new key here: 
https://luci-milo.appspot.com/raw/build/logs.chromium.org/flutter/led/ricardoamador_google.com/bc5d1e66627ad6b9f58d2dc85b7524692520e83858132b9e92ff8d3166b1b7e5/+/build.proto

And the bot is now generating pull requests: https://github.com/flutter/flutter/pull/135315

*List which issues are fixed by this PR. You must list at least one issue.*
Fixes https://github.com/flutter/flutter/issues/131968

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

.ci.yaml

@@ -256,7 +256,6 @@ targets:
     presubmit: false
     recipe: pub_autoroller/pub_autoroller
     timeout: 30
-    bringup: true # https://github.com/flutter/flutter/issues/131968
     enabled_branches:
       # Don't run this on release branches
       - master