Comment 4 for bug 1711735

This was fixed for xenial in bug 1513529 and bug 1679252. (Specifically, the former refactored sources.list generation significantly and in a way which broke non-x86 architectures, and the latter remedied that.)

Unfortunately, the changes to sources.list generation were fairly major, so I don't think it would be appropriate to pull them all the way back to trusty; we should find a less invasive fix.