We have disabled CircleCI and are using GitHub Actions for automated
testing.
docs: Changed context from CircleCI to Github Actions and wrote
some documentation specific to GH Actions.
tools: Replaced env checks for CIRCLECI with GITHUB_ACTION.
README: Use GitHub Actions build status badge.
This allows to run scripts between extraction and install
process.
It will be used to restore npm caches for production install jobs.
We extract the tarball in the working directory so that yarn.lock and
package.json are available to restore cache.
(And also so the path is deterministic).