MT#6809 Explicitely invoke piuparts with --mirror option to support jessie

piuparts uses our internal mirror by default:

| Err http://debian.sipwise.com jessie/main amd64 Packages
|   404  Not Found [IP: 77.244.249.93 80]
| Err http://debian.sipwise.com jessie/contrib amd64 Packages
|   404  Not Found [IP: 77.244.249.93 80]
| Err http://debian.sipwise.com jessie/non-free amd64 Packages
|   404  Not Found [IP: 77.244.249.93 80]

But we don't provide jessie on it (yet), so explicitely use
piuparts --mirror option to point it to the according place.