Temporary: prefer kmod executables over busybox ones (#1060134).

kmod's modprobe works fine since xz support got added (30+20230601-2.1)
while current busybox (1:1.36.1-6) can't load modules. Use executables
from the former (found in /usr/sbin), dropping those from the latter
(found in /sbin), fixing the FTBFS in merge-usr.

Once we're sure this is the way to go, we can ask the maintainer of the
relevant package to drop redundant executables.