Ensure mac builder has graphics device

Author: hadley

.github/workflows/R-CMD-check.yaml

@@ -42,6 +42,10 @@ jobs:
 
       - uses: r-lib/actions/setup-pandoc@v1
 
+      - name: Install xquartz
+        if: runner.os == 'macOS'
+        run: brew install --cask xquartz
+
       - uses: r-lib/actions/setup-r@v1
         with:
           r-version: ${{ matrix.config.r }}