CI: Update for detached HEAD state

.github/workflows/dev-build-pr.yml

@@ -35,7 +35,8 @@ jobs:
         if : ${{ github.event_name == 'pull_request_target' }}
         shell: pwsh
         run: |
-          $previous_commit = git rev-parse 'HEAD^'
+          $commits = git log -n 2 --pretty=format:%H
+          $previous_commit = $commits[1]
           if ($previous_commit -ne "${{ github.event.pull_request.head.sha }}") {
             throw "The pull request head commit has changed since the event was triggered."
           }