Set ARCH to aarch_64 for macOs.

Author: kannanjgithub

buildscripts/kokoro/macos.sh

@@ -24,5 +24,6 @@ export -n JAVA_HOME
 brew install --cask temurin@8
 export PATH="$(/usr/libexec/java_home -v"1.8.0")/bin:${DOWNLOAD_DIR}/cmake-${CMAKE_VERSION}-macos-universal/CMake.app/Contents/bin:${PATH}"
 brew install maven
+export ARCH=aarch_64
 
 "$GRPC_JAVA_DIR"/buildscripts/kokoro/unix.sh