[check-links] Diff against common ancestor with origin/main (#11443)

* [check-links] Diff against common ancestor with origin/main

* Use github variable instead of HEAD

.github/workflows/check-links.yaml

@@ -19,7 +19,7 @@ jobs:
       - name: Get changed files
         id: changes
         run: |
-          echo "::set-output name=md::$(git diff --name-only --diff-filter=ACMRTUXB origin/${{ github.event.pull_request.base.ref }} ${{ github.event.pull_request.head.sha }} | grep .md$ | xargs)"
+          echo "::set-output name=md::$(git diff --name-only --diff-filter=ACMRTUXB $(git merge-base origin/main ${{ github.event.pull_request.head.sha }}) ${{ github.event.pull_request.head.sha }} | grep .md$ | xargs)"
   check-links:
     runs-on: ubuntu-latest
     needs: changedfiles