ci: use tag v2 when using checkout action

Using v2 automatically fetches the latest minor patch for v2, meaning
free bug fixes without needing to manually update the tag version.

.github/workflows/commitlint.yml

@@ -9,7 +9,7 @@ jobs:
     env:
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:
-      - uses: actions/checkout@v2.3.1
+      - uses: actions/checkout@v2
         with:
           fetch-depth: 0
           ref: ${{ github.event.pull_request.head.sha }}