kbuild: buildtar: always make modules_install

It is done for the same reasons as 4243afdb

 does it for builddeb:
always runs make modules to install modules.builtin* files, which are
needed for e.g. initramfs-tools or LTP testing tool.

Signed-off-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>