`lookup_result_architecture` returning a string with multiple architectures instead of an architecture (for a DebianUpload)

In !1972 (comment 620971)

(it will be merged and be part of DebDiffWorkflow used as lookup_result_architecture(result).split())

TBD: perhaps lookup_result_architecture should return multiple architectures, or do not accept a DebianUpload and have a different call (lookup_result_architectures) for artifacts that might contain different architectures.