Trigger CI on pull_request given security problems of pull_request_target

[r-n-o]

In #35 I switched the main CI workflow to trigger on pull_request_target to make external contributions easier. The intent was to let PRs from forks run CI, with approval.

Turns out the approval part doesn't work and external contributors can run CI without an approval. The secret in this workflow is inconsequential but it's still bad practice to leave the current workflow as-is.

Looking at the Github documentation, it clearly states:

This event runs in the context of the default branch of the base repository, rather than in the context of the merge commit, as the pull_request event does. This prevents execution of unsafe code from the head of the pull request that could alter your repository or steal any secrets you use in your workflow. This event allows your workflow to do things like label or comment on pull requests from forks. Avoid using this event if you need to build or run code from the pull request.

[cr-tk]

Approving, as per internal discussion.