Add a flag to only download tarballs for easier caching without setting up chroot

[onlyservers]

This PR adds a new flag to make it easier to download tarballs without setting up the chroot just yet.

This can make docker-based workflows easier to use since this step can happen at image build time to cache the tarballs locally.

[igorpupkinable]

@SynologyOpenSource is there any plan for the MR?