chore: ensure yarn lockfile is stable in CI.
chore: ensure yarn lockfile is stable in CI.

> If you need reproducible dependencies, which is usually the case with
> the continuous integration systems, you should pass
> `--frozen-lockfile` flag.

from: https://classic.yarnpkg.com/en/docs/cli/install/

heroku/cli,heroku/cli,heroku/cli,heroku/heroku-cli,heroku/cli,heroku/heroku-cli,heroku/heroku-cli,heroku/heroku-cli