debian_pipeline doesn't allow source_artifact to be a debian:upload, due to sbuild workflow restrictions

The documentation of the debian_pipeline workflow describes source_artifact as "the debian:source-package or debian:upload artifact representing the source package to test". But if you try it with a debian:upload, you get this in the server debug log:

2024-11-14 13:17:46,320 [base/6054/140358219640896] WARNING: Error running work request Workflow/sbuild (865): orchestrator failed: source artifact is type 'debian:upload' instead of debian:source-package
2024-11-14 13:17:46,320 [base/6054/140358219640896] WARNING: Error running work request Workflow/debian_pipeline (864): orchestrator failed: (<WorkRequest: 865>, "orchestrator failed: source artifact is type 'debian:upload' instead of debian:source-package")

I think ideally the sbuild workflow would accept a debian:upload containing a source package (analogous to *_source.changes) as well as a debian:source-package (analogous to *.dsc).