CI: try the official Coveralls action

<!-- Provide a general summary of your proposed changes in the Title field above --> <!-- The title should have the following form: 'Type: Subject' with 'type' in [Build,Chore,CI,Deprecate,Docs,Feat,Fix,Perf,Refactor,Revert,Style,Test] --> ### Description Test if the official Coveralls Github action works better. ### Checklist <!-- Go over following points. Check them with an `x` if they do apply (they turn into clickable checkboxes once the PR is submitted, so no need to do everything at once). --> This pull request is: - [ ] A documentation / typographical error fix - Good to go, no issue or tests are needed - [ ] A short code fix - [ ] Please include: `Fixes: #<issue number>` in the description if it solves an existing issue (which must include a complete example of the issue). - [ ] Please include tests that fail with the `main` branch and pass with the provided fix. - [ ] A new feature implementation or update an existing feature - [ ] Please include: `Fixes: #<issue number>` in the description if it solves an existing issue (which must include a complete example of the feature). - [ ] Please include tests that cover every lines of the new/updated feature. - [ ] Please update the documentation to describe the new/updated feature. <!-- **Have a nice day!** -->