package/libubox: fix lua module install path

The LUAPATH variable is only used when installing the lua module, and
cmake knows very well how to do out-of-tree isntalls, so there is no
reason to include the staging path in LUAPATH, which will cause our
post-install sanity checks to kick in and whine:

    libubox: installs files in /home/lhk/workspace/orangepi/host/usr/a
    rm-buildroot-linux-gnueabihf/sysroot//home/lhk/workspace/orangepi

As for libuci, just pass the runtime LUAPATH.

Fixes #9856

Reported-by:  <linhuikui@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>