this lets us *dis*able filesystem explicitly, as is required by e.g. the
zfs-less installer images. currently that specifically is only easily
possible by adding an overlay that stubs out `zfs`, with the obvious
side-effect of also removing tooling that could run without the kernel
module loaded.
5.7+ comes with a native exfat implementation, exfatprogs should be used instead.
The exfat package puts a "mount.exfat" binary in the path, which causes
mount to prefer the FUSE version to the non-fuse one. There's no way to
disable the binary, so switch to exfatprogs.
