re-enable linux packages-autoroller (#160266)

I fixed the autoroller in https://github.com/flutter/flutter/pull/160059
but forgot to mark the shard non-bring-up. Note, it's currently failing
in post-submit, but only because bringUp shards run in the staging pool,
which does not have access to the GitHub access token necessary for the
roller to operate.

.ci.yaml

@@ -392,7 +392,6 @@ targets:
 
   - name: Linux packages_autoroller
     presubmit: false
-    bringup: true # https://github.com/flutter/flutter/issues/160055
     recipe: pub_autoroller/pub_autoroller
     # This takes a while because we need to fetch network dependencies and run
     # Gradle for every android app in the repo