fix(pr-validation): fetch PR head refs instead of branch name

Prior --depth=1 fetch of $HEAD_REF fails because shallow clone
doesn't bring in the PR head branch as a ref.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/pr-validation.yaml

@@ -17,7 +17,7 @@ jobs:
         run: |
           git clone --depth=1 "https://x-access-token:${{ secrets.GITEA_TOKEN }}@git.farh.net/${{ github.repository }}.git" .
           git fetch origin "$BASE_REF" --depth=1
-          git fetch origin "$HEAD_REF" --depth=1
+          git fetch origin +refs/pull/*/head:refs/pull/*/head --depth=1
           git checkout "${{ github.sha }}"
 
       - name: Install actionlint