build: fix commit linter on unrebased PRs

The commit linter was checking out the PR HEAD commit instead of
merge/rebase commit, causing it to fail for any PRs that were not
rebased on our default branch. Removing `ref` should fix the issue.

PR-URL: #39121
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>

Author: mmarchini

Diff for: .github/workflows/commit-lint.yml

@@ -8,7 +8,6 @@ jobs:
     steps:
       - uses: actions/checkout@v2
         with:
-          ref: ${{ github.event.pull_request.head.sha }}
           # Last 100 commits should be enough for a PR
           fetch-depth: 100
       - name: Use Node.js 12