`@commitlint/config-lerna-scopes` package depends on lerna but doesn't have `lerna` listed in its dependencies, so the action was failing. Since this action doesn't run in the repo's context, we need to install lerna as a dependency to make it work.
For any event that is not of type `pull_request`, we consider there will be a `GITHBU_SHA` defined and run lint against only this specific commit
sha.
This supports not only events of type `pull` but also other events that provide us a `GITHBU_SHA`, like `check_suite`.
