reprotest: use a virtual server backend other than null

Currently, reprotest uses the default null backend, that seems OK for unstable. However, for other releases, specially old stable releases, this causes issues, especially with dependencies and incompatibilities. See for example this in the LTS Team's pipeline fork: https://salsa.debian.org/lts-team/pipeline/-/jobs/5694530#L2141

There are probably similar issues with packages from bookworm or bullseye.