Fix codecov covergae reports

[olegbespalov]

What?

It seems like our code coverage reports are going to the following: https://app.codecov.io/github/k6io/k6/commits

instead of the

https://app.codecov.io/gh/grafana/k6/commits

And we also don't use an existing GitHub Action https://github.com/codecov/codecov-action

So, we should fix that.

Why?

We no longer use the https://app.codecov.io/github/k6io/k6/commits