Specify head ref to checkout in dependabot PR workflow (#1653) (#1654)

(cherry picked from commit 8f30e5f)

Signed-off-by: Thomas Farr <tsfarr@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: opensearch-trigger-bot[bot]

.github/workflows/dependabot_pr.yml

@@ -21,6 +21,8 @@ jobs:
         uses: actions/checkout@v4
         with:
           token: ${{ steps.github_app_token.outputs.token }}
+          ref: ${{ github.event.pull_request.head.ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
 
       - name: Update the changelog
         uses: dangoslen/dependabot-changelog-helper@v4