zero_coverage report should use covdir

[La0]

The zero coverage reports generated on the bot use the coveralls+ format, which should not be used anymore.

[La0]

1. build the main covdir report using current workflow
2. use that report and filter it to only keep the zero coverage files
3. Either:
   3.a ship the resulting covdir report as an artifact and patch the frontend to use it (if that report is small enough)
   3.b convert the covdir report to the current format, no changes needed on frontend