Resolve BuildID variance supported by extended Salsa-CI reprotesting.

The --variations argument for reprotest resets previously-specified variations, and when a value is provided (meaning that the default does not apply) that does not start with all, the subsequent comma-separated items vary from an initially-empty set.

Please refer to this reproducible-builds.org mailing list thread for additional information.

Also, reprotest expects build_path (with an underscore) and not build-path (with a hyphen).

Opening this merge request is expected to result in a failure of the reprotest pipeline job due to bug #1003923 - a patch from that bugreport may be applied and should resolve the problem.

Closes #1003923.