ci: Do not fail ci if Coverage reporting fails

[maxreichmann]

As seen in this GH Actions run, the Coveralls servers can be down very easily.

This adds continue-on-error to the coverage upload step too (previously only on coverage calculation steps)

[maxreichmann]

Outcome of discussion (Dec 15, @d3xter666 & @maxreichmann):

Since we don't want to miss if the Coverage actually went down and get notified about it, the approach is as follows:
We're keeping the flag for the parallel steps ("Coverage / Check and Upload (packages/...)") and not adding it to the upload step ("Coverage / finish"). When the servers are up, we can see the coverage percentage without problems. When the servers are down, we can easily calculate the coverage locally and are still be able to merge.

Closing.