Fix the build after 40114796f0

That patch added new flags but gRPC is still built with GNU C++
compiler. If we're trying to build both gRPC and LLVM with the same set
of flags, we should enforce additional CMake flags for gRPC.

.github/workflows/autobuild.yaml

@@ -118,6 +118,7 @@ jobs:
         "-DCMAKE_C_FLAGS_RELEASE=${{ matrix.config.cflags }}"
         "-DCMAKE_CXX_FLAGS_RELEASE=${{ matrix.config.cflags }}"
         ${{ matrix.config.grpc_cmake }}
+        ${{ matrix.config.cmake }}
 
         ninja -C grpc-build install
     - name: Fetch target commit