The piuparts worker task should generate a debian:piuparts artifact

The piuparts task is the only task run by the qa workflow that doesn't generate a proper artifact describing the result. It would make sense to generate such an artifact so that it can be stored in the debian:qa-results collection together with appropriate meta-information such as the piuparts version that was used to run the test, and the list of binary packages that have been tested.

Those meta-information make it possible to know if a previously-stored piuparts analysis is still up-to-date or not.