ci: right sha for authors check

.github/workflows/authors.yml

@@ -23,7 +23,7 @@ jobs:
       - name: Export known authors from master branch
         run: git log --format="%an <%ae>" origin/master | sort | uniq > authors.txt
       - name: Export authors from new commits
-        run: git log --format="%an <%ae>" origin/${GITHUB_BASE_REF}... | sort | uniq > commit-authors.txt
+        run: git log --format="%an <%ae>" ${{ github.event.pull_request.base.sha }}... | sort | uniq > commit-authors.txt
       - name: Check new authors
         run: |
           touch new-authors.txt