ci: Generate code coverage for all test jobs

We have an expilicit "coverage" job that computes test coverage. Now that we have started to compute code coverage in our tests by default though it doesn't make much sense anymore to treat this job specially.

Refactor our CI pipeline to extract code coverage for all jobs. This gives us a better global picture of which jobs cover what parts of our codebase. Remove the Makefile target that drove this as it is likely no longer of use to anybody.