Don't overwrite WATCHLISTS each time with a checkout from the latest
release branch as that means it will never pick up changes from
master.

No-Try: true
Bug: chromium:832032
Change-Id: I3a9231369caa9a6591acb9b7f0c76dc031ab9178
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1926029
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65063}