build: fix commit linter on unrebased PRs

The commit linter was checking out the PR HEAD commit instead of
merge/rebase commit, causing it to fail for any PRs that were not
rebased on our default branch. Removing `ref` should fix the issue.

PR-URL: https://github.com/nodejs/node/pull/39121


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>