How can I only run a step if the workflow is running in the main repository, and not within a fork?

I would have thought that
if: github.repository == 'octo-org/octo-repo-prod' would do it, but it does not – and while if: ${{ github.event.pull_request.head.repo.full_name == 'octo-org/octo-repo-prod' }} does work for PRs, it does not work for merges or scheduled builds.