Bring back code coverage per PR

[gemelen]

Investigate and fix an issue with code coverage not being posted in PRs.
Coverage is sent on each CI run, but isn't calculated on codecov side and not posted.

[gemelen]

That notification that "CI failed" is quite suspicious here. I suggest that something changed configuration-wise on Codecov side and for 8 or so months this repo has no coverage reports thereafter.

[armanbilge]

I plan to add an scoverage integration to sbt-typelevel in the near future. Once that lands it should be very very easy to enable.