dracut: fix boot on non-zfs-root systems

Simply prevent overwriting root until it needs to be overwritten.

Dracut could change this value before this module is called, but won't
change the kernel command line.

Reviewed-by: Andrew J. Hesford <ajh@sideband.org>
Signed-off-by: Toyam Cox <vaelatern@voidlinux.org>
Closes #13592