ci: add code coverage via Codecov

ci: add code coverage via Codecov

Add a simple code coverage pipeline based on the CodeQL one.

Should give us pretty reports in the PRs and web UI with the results. We
can consider having HTML reports on our website (once it's up that is)
in addition or instead of at a future point.

Closes: https://github.com/kmod-project/kmod/issues/61
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/229
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>