Run the zero coverage generation in the repo hook rather than the cron one

[marco-c]

This way the cron hook will be very short (as we'll also be able to stop cloning m-c and retrieving coverage artifacts) and we can run it multiple times per day (to catch newer m-c runs that we didn't ingest yet earlier).

1. Run the zero coverage task in the repo hook too;
2. Modify users of the zero coverage artifact to use the new URL;
3. Remove zero coverage generation from the cron hook.