Overlayns broken in Steam flatpak

[Orochimarufan]

Using Kawariki from the Steam flatpak without KAWARIKI_NO_OVERLAYNS no longer works. Native (as in from distro packages) Steam installations don't seem to be affected.

This may be caused by Steam, the Steam flatpak, or core flatpak/bwrap changes.

Ideally, overlayns should be replaced by a well-known tool (a la unshare(1)), but I haven't found one that supports setting up arbitrary mounts before dropping (userns) privileges.