CI: Improve codecov reporting
Codecov is adding two comments to our PRs after the tests are successfully run. Which makes us receive two messages. Personally I don't find these useful. I see that other projects have codecov in the checks, next to the builds (expand the "All checks have passed" in https://github.com/xhochy/fletcher/pull/74 to see an example).

Is other people finding those comments/messages useful? Would be enough for you to just have the info in the list of checks and no emails?

xref #12634