github: [renderdiff] ensure right clang (#9816)

For renderdiff, we compile mesa using brew's llvm/clang, but
we compile filament with the system clang (AppleClang).

This is to prevent compiling filament with brew's clang (which
can cause unexpected compilation error).

Author: poweifeng

.github/actions/renderdiff-generate/action.yml

@@ -8,9 +8,15 @@ runs:
     - uses: ./.github/actions/get-mesa
     - uses: ./.github/actions/get-vulkan-sdk
     - name: Prerequisites
+      if: runner.os == 'macOS'
       run: |
         # Must have at least clang-16 for a webgpu/dawn build.
         sudo xcode-select -s /Applications/Xcode_16.2.app/Contents/Developer
+        # this enforces the gltf_viewer build to use apple clang as oppose the brew clang used for get-mesa
+        echo "CC=/usr/bin/clang" >> $GITHUB_ENV
+        echo "CXX=/usr/bin/clang++" >> $GITHUB_ENV
+        echo "/usr/local/bin" >> $GITHUB_PATH
+        echo "/usr/bin" >> $GITHUB_PATH
       shell: bash
     - name: Generate images
       run: |