Update actions/checkout action to v3 (postcss#430)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

Author: renovate[bot]

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - run: git config --global core.autocrlf input
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Use Node.js ${{ matrix.node }}
       uses: actions/setup-node@v3
       with: