Do not upload coverage results to Codecov when run in a fork

[jdblischak]

The coverage workflow always fails when I sync my fork because the upload to Codecov fails. Thus I want to disable the upload step unless running on the main repository.

I think I found a good workaround: if: github.event.repository.fork == false. It skipped the upload step when I did a test run from my fork.

My only remaining doubt is what will happen in a Pull Request from a fork (like this one). If the coverage upload step runs for this PR, that is great, and we should merge this change. However, if the the coverage upload is skipped for this PR, I'll have to revert to the less generic if: github.repository == 'Merck/simtrial'

Note that I also added the manual trigger workflow_dispatch: to make it easier to run the coverage workflow from a feature branch.

[jdblischak]

Shoot. It didn't upload the coverage results (https://github.com/jdblischak/simtrial/actions/runs/12876137723/job/35898635722)

[nanxstats]

Nice, @jdblischak do you think it's useful to also contribute this condition to upstream r-lib/actions once it's confirmed working? Feels to me like a fairly common use case that should be safeguarded.