message: [ci] Use PR head commit instead of merge commit
  GitHub defaults the checkout action to the merge commit, which breaks all the
  assumptions of the verifier.
diff: @@ -13,6 +13,8 @@ jobs:
  steps:
  - name: checkout
  uses: actions/checkout@v2
  + with:
  + ref: ${{ github.event.pull_request.head.sha }}
  - name: fetch base ref
  id: fetch-base-ref
  uses: ./.github/actions/pr-base-ref
  