Use actions/checkout for PR (#2804)

.github/workflows/mingw.yml

@@ -42,14 +42,11 @@ jobs:
           git clone --single-branch --shallow-since=yesterday https://github.com/ruby/ruby src
           git -C src reset --hard "$GITHUB_SHA"
         if: github.event_name == 'push'
+      # It's hard to propagate `env` to this workflow's shell environment to checkout PR. Using unstable actions/checkout as a workaround.
       - name: Checkout a pull request
-        run: |
-          git clone --single-branch --shallow-since=yesterday "--branch=$GITHUB_BRANCH" "https://github.com/${GITHUB_REPO}" src
-          git -C src reset --hard "$GITHUB_REV"
-        env:
-          GITHUB_REV: ${{ github.event.pull_request.head.sha }}
-          GITHUB_BRANCH: ${{ github.event.pull_request.head.ref }}
-          GITHUB_REPO: ${{ github.event.pull_request.head.repo.full_name }}
+        uses: actions/checkout@v2
+        with:
+          path: src
         if: github.event_name == 'pull_request'
       - run: ./src/tool/actions-commit-info.sh
         shell: bash