The unionfs-fuse package that we use for the config filesystem has been removed from Trixie.
We can start building it ourselves even before the move to Trixie, and then remove it if the future Trixie-based 1.6 will no longer need the legacy config subsystem.
The change can be imported from https://github.com/vyos/vyos-build/pull/978/commits/dee18458feb461fe8b7d95276646f767f0525353