GHA: need to install libpng on macOS

Author: gaborcsardi

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

@@ -52,6 +52,11 @@ jobs:
           http-user-agent: ${{ matrix.config.http-user-agent }}
           use-public-rspm: true
 
+      - name: Install libpng on macOS
+        if: runner.os == 'macOS'
+        run: |
+          brew install libpng
+
       - uses: r-lib/actions/setup-r-dependencies@v2
         with:
           extra-packages: any::rcmdcheck