Fix CI for external contributed PRs

Our CI workflow doesn't expose env. variables for security reasons when running PRs coming from forked repositories. We need to optimise our workflow to not execute steps that require them. Example of failed build [here](https://circleci.com/gh/mapbox/mapbox-plugins-android/3324?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link).

cc @mapbox/maps-android

Start with the CI workflow and the failed CircleCI build linked in the issue. Identify steps that require environment variables unavailable to forked PRs; done means external contributed PRs skip those steps and the workflow completes successfully.

Written by the indexing model from the issue text.