Debusine requires piuparts 1.3

Debusine passes a bare system.tar with no compression to piuparts. That's a new feature to piuparts that I introduced in version 1.3, which is available in trixie, but not in bookworm or earlier. In earlier releases, piuparts expects to have a gzip-compressed --basetgz.

In principle, this is fine as we run piuparts inside an environment and we may just provide a sufficiently recent environment. Except that the debian_pipeline does not allow setting a suitable environment for piuparts and hence we cannot successfully run piuparts via the debian_pipeline when using the pipeline for releases older than trixie. Can we make this work somehow?