This is done by requesting the refs and shas for the PR when the
env.sh script is run.  Additionally, the env.sh script is now setup
to write all of the environment variables created to a cache file
and subsequent loads of the environment load the values from there.

The get-refs-and-shas-for-target.js script now also first attempts
to load the refs and shas from an environment variable before
falling back to requesting from github via the API.

PR Close #36207