upload-to-experimental is not functioning, likely due to a missing experimental sbuild

The workflow in question is https://debusine.debian.net/debian/developers/work-request/89517/ and notably, is the first one to use upload-to-experimental.

This fails with "Orchestrator failed: None of the workflow architectures are supported by this package: workflow: all package: any".

Per IRC, the issue is likely due to a missing sbuild for experimental.