GH Actions shortlog: check out head ref

This avoids the action failing on PRs due to the merge commit checked out by default having different author details to the locally pushed changes by that contributor

.github/workflows/shortlog.yml

@@ -17,6 +17,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           fetch-depth: 0  # need to fetch all commits to check contributors
+          ref: ${{ github.head_ref }}
 
       - name: Check CONTRIBUTING.md
         uses: cylc/release-actions/check-shortlog@v1