code coverage isn't measured and reported

[fasmat]

The PoST repo is missing a coverage job that uploads code coverage to codecov.io. As a reference the job from poet can be used:

https://github.com/spacemeshos/poet/blob/6aa1e97ee23911bae6292c737345e2b4c9cd468f/.github/workflows/ci.yml#L94-L111

and

https://github.com/spacemeshos/poet/blob/develop/.github/codecov.yml