Fix the DCM workflow (#5925)
Follow up to https://github.com/flutter/devtools/pull/5886
Currently the DCM workflow is running against the last commit on the master branch, not on the PR branch
From reading https://github.com/orgs/community/discussions/25191#discussioncomment-3246770, I believe `github.event.pull_request.head.sha` is what we want.
