This PR speeds up PR builds by more than a minute if there are tests in the package and also makes coverage reporting reliable again:

• Fixes the previously broken coverage. Coverage data was not reliably uploaded anymore, as codecov requires an organization token or else a global GH token is used. The covr::codecov call uses an old API that does not support organization tokens and thus we have also used the global GH token. We now use the codecov upload action with our organization token. To enable this, I needed to add a custom report format. In case Adds the simple codecov format to make using the codecov action easier r-lib/covr#616 is merged, we could replace this.
• Limits coverage calc to main branches. We could simplify check.yml by introducing a separate coverage-reporting.yml that only execute coverage and only does so on the main branch. Not sure whether it is worth the effort yet to split up check.yml, given that it is somewhat small still.