ci: fix version check CI job (#3032)

Part of the job checks out the pull request (this isn't done
automatically because this runs with `pull_request_target` which checks
out the base by default).

That step was using `merge_commit_sha` which is not correct (this still
checks out the base). The fix instead checks out `head.sha` which is
correct.

It is not possible to get a `merge_commit_sha` (there is a request for
this [here](actions/checkout#518)) but
`head.sha` should be good enough for our purposes.

.github/workflows/pr-title.yml

@@ -36,7 +36,7 @@ jobs:
         with:
           # pull_request_target checks out the base branch by default, not
           # the PR branch.
-          ref: "${{ github.event.pull_request.merge_commit_sha }}"
+          ref: "${{ github.event.pull_request.head.sha }}"
           path: pr
       - uses: actions/setup-python@v5
         with: