{kokoro} Add bazel pinning support to High Sierra images (#6498)
{kokoro} Add bazel pinning support to High Sierra images (#6498)

This PR fixes our xcode10.1.0 presubmit jobs.

## Context

Kokoro's High Sierra machines do not yet include a `use_bazel.sh` tool, which our `.kokoro` script uses to pin the bazel version to 0.20.

## The problem

Our kokoro High Sierra jobs are failing because they are unconditionally attempting to run `use_bazel.sh`.

## The fix

We've explicitly added `use_bazel.sh` to our High Sierra images now. The script is placed in a specific location, however, so this PR updates the bazel pinning logic to make use of the script if it's available in that specific location.
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios