[ci.yaml] Promote roller to blocking

[CaseyHillers]

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[christopherfujino]

LGTM

[zanderso]

I am not comfortable with this. The recipe that does this pulls many repos from head, and so could therefore cause the Engine tree to turn red due to commits to other repos.

[zanderso]

/cc @godofredoc