inst_upgrade_urls.rb: set the target distribution (bnc#881320)

to not ignore the SCC online repositories in libzypp

- 3.1.92
This is only set if `!@repo_files_to_remove.empty?` but what if there are none? In this case it would  NOT be called and `"target_distro"` would not be set. I'm not sure whether it's enough.