New style steps should use `run` instead of `start`. Convert BlockPullRequest to a new-style step.
<rdar://problem/104453787>
Pull request: https://github.com/WebKit/WebKit/pull/8863
Committed 259278@main (4e32b6b25e79): <https://commits.webkit.org/259278@main> Reviewed commits have been landed. Closing PR #8863 and removing active labels.