Fix flatpak enter being multi-threaded and failing
Fix flatpak enter being multi-threaded and failing

flatpak enter must not be multi-threaded because it calls setns. This
was causing a problem because g_vfs_get_default spawns threads in some
environments.

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak