add option for post job cleanup of creds

[marengaz]

TL;DR

option for post job cleanup of creds

Design

Action YAML

      - uses: google-github-actions/setup-gcloud@master
        with:
          project_id: my-project
          service_account_key: ${{ secrets.MY_SA_KEY }}
          export_default_credentials: true
          post_job_credential_cleanup: true

a rudimentary way to do this would be something like:

rm -f "$GOOGLE_APPLICATION_CREDENTIALS"
unset GOOGLE_APPLICATION_CREDENTIALS

Additional information
gives confidence that if a worker is reused, the creds wont still be hanging around

[uportalis]

I think such clean-up should be enabled by default.