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.
Support for ZFS, while desirable, is problematic with newer kernel
releases. The stable ZFS release seldom supports the current newest
kernel version, and this makes the new_kernel iso basically useless as
it cannot be published, and is not often built with new kernel releases.
This uses a dirty workaround to work around the fact it is impossible to
remove a list item from a modules system list type. Since ZFS support is
conditional to being supported on the current platform, we can fake ZFS
not being supported *for the no-zfs build only*. This overlay is only
added when evaluating the iso, nothing else.
