SLR 1.0: Assume that steam-runtime.tar.xz is a monolithic tarball

In older Steam releases this was distributed broken up into pieces (steam-runtime.tar.xz.part*), but since beta 1705630720 (2024-01-19) and GA 1714854927 (2024-05-04) it's a single large file. Adjust our fallback code path to take this into account.

steamrt/tasks#490

/cc @denittis @refi64 @timo