message: api: fix api_mirror.sh CHECKOUT_DIR dependency.
diff: @@ -5,6 +5,8 @@ set -e
  # TODO(htuch): Remove this once we've verified this script works.
  set -x
  +CHECKOUT_DIR=../data-plane-api
  +
  if [ -z "$CIRCLE_PULL_REQUEST" ] && [ "$CIRCLE_BRANCH" == "master" ]
  then
  echo "Cloning..."
  