Use event 'pull_request_target' only on forks

I currently use pull_request_target successfully for all types of PRs (both from branches in forked repos as well as from branches in the original repo). For PRs changing the GH workflow file itself this obviously does not work well (as it always acts on the target branch workflow). I would therefore like to use pull_request but only for PRs from branches of the original repo and keep on using pull_request_target for all other PRs.
Is there any solution how to combine both approaches?