ci: workflow should not fail when coveralls is down

### Description / Beschreibung

There seem to be some failures with coveralls lately, https://status.coveralls.io/history.

Now, our CI runs fail even when coveralls sends any of the 500 errors.
Probably it can be ignored on error.
 
* https://github.com/rdmorganiser/rdmo/actions/runs/17856551074
* https://github.com/marketplace/actions/coveralls-github-action#inputs