{CI only} Use pull_request_target instead of pull_request

[levb]

We are having an issue running coverage upload from forked repo PRs. This should solve it, I think.

Testing for now from my own fork.

[kozlovic]

A quick look at this event seems to show that there may be some security risks. So unless you perfectly understand the context and are positive that this does not apply, I would not make this change: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target (look for the Warning section and following link).

[levb]

@kozlovic I am still experimenting with this one

[kozlovic]

@levb Sorry, I thought that I was asked to review, my mistake.

[levb]

I am just going to take codecov out of the forked repo PR flow for now. It appears that the codecov people are working on the issue, and there is a workaround but it's kludgy. Will close this PR and open another one to remove codecov from fork PRs for now