Make fetching of backports and proposed-updates repositories optional

The Apertis pipeline imports backport and/or proposed-updates versions by default, this is not needed and leads to miss some security updates. Because, importing versions from backports and proposed-updates is not part of our standard workflow, this should only be optional to avoid confusing the pipeline.

See infrastructure/ci-package-builder!316 (merged) for the ci-package-builder part.

https://phabricator.apertis.org/T9538