Try to fix variable reference on Windows (#2361)

https://github.com/ruby/ruby/runs/192869165

.github/workflows/windows.yml

@@ -33,16 +33,10 @@ jobs:
         run: choco install winflexbison3
       # Not using official actions/checkout because it's unstable and sometimes doesn't work for a fork.
       - name: Checkout ruby/ruby
-        run: git clone --depth=50 https://github.com/ruby/ruby . && git reset --hard "$GITHUB_SHA"
-        env:
-          GITHUB_SHA: ${{ github.sha }}
+        run: git clone --depth=50 https://github.com/ruby/ruby . && git reset --hard ${{ github.sha }}
         if: github.event_name == 'push'
       - name: Checkout a pull request
-        run: git clone --depth=50 "--branch=$GITHUB_BRANCH" "https://github.com/${GITHUB_REPO}" . && git reset --hard "$GITHUB_REV"
-        env:
-          GITHUB_REV: ${{ github.event.pull_request.head.sha }}
-          GITHUB_BRANCH: ${{ github.event.pull_request.head.ref }}
-          GITHUB_REPO: ${{ github.event.pull_request.head.repo.full_name }}
+        run: git clone --depth=50 --branch=${{ github.event.pull_request.head.ref }} https://github.com/${{ github.event.pull_request.head.repo.full_name }} . && git reset --hard ${{ github.event.pull_request.head.sha }}
         if: github.event_name == 'pull_request'
       - name: configure
         run: |