Merge pull request #17682 from apolcyn/attempt_homebrew_fix

Attempt to fix homebrew installation problem

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -38,6 +38,8 @@ if [ -n "$KOKORO_GITHUB_PULL_REQUEST_NUMBER" ]; then
 fi
 
 set +ex  # rvm script is very verbose and exits with errorcode
+# Advice from https://github.com/Homebrew/homebrew-cask/issues/8629#issuecomment-68641176
+brew update && brew upgrade brew-cask && brew cleanup && brew cask cleanup
 source $HOME/.rvm/scripts/rvm
 set -e  # rvm commands are very verbose
 time rvm install 2.5.0