message: Install proto for python in the cross language MacOS test.
diff: set -euo pipefail
  cd ${KOKORO_ARTIFACTS_DIR}/git/tink
  -
  cd tools
  +
  +# TODO(b/155225382): Avoid modifying the sytem Python installation.
  +pip3 install --user protobuf
  +
  use_bazel.sh $(cat .bazelversion)
  time bazel build -- ...
  time bazel test -- ...
  