Update prepare_build_macos_rc

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -26,8 +26,14 @@ ulimit -a
 # - only add stuff that you absolutely need for your builds to work (add comment to explain why its needed)
 
 # Disable HOMEBREW update to avoid new updates which potentially have problems.
+# Brew packages installed when Kokoro image was built tend to have less conflict.
 export HOMEBREW_NO_AUTO_UPDATE=1
 
+# Dump the brew configuration for debugging just in case. Check "Core tap HEAD" field
+# because it should be the same as below unless it's been updated.
+# - Kokoro MacOS Mojave: 27fa87c94a6cf7be40fc8f8fc96bc7c387b7781e
+brew config
+
 # Add GCP credentials for BQ access
 pip install --user google-api-python-client oauth2client six==1.15.0
 export GOOGLE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/GrpcTesting-d0eeee2db331.json