Allow running GitHub Actions or workflows on pull_request event

[stevepiercy]

GitHub Actions (workflows) currently run only when someone opens a branch in the plone/volto repository, and not from forks. Should we open that up?

The effort to do so is minimal. It means editing our existing workflows to execute on a pull_request event instead of a push.

Currently the push event only is an annoying barrier, as external branches do not run workflows. We can still require the Plone Contributor Agreement, and @mister-roboto happily nags the contributor to do so. The risk is minimal.

The downside is that external forks typically are not writeable except by the owner, and thus do not allow others to amend the pull request.

I'd suggest this as a topic of discussion at Alpine City Sprint.