dev-lang/lua: fix slotted lua

dev-lang/lua: fix slotted lua

Command changing pkg-config file to appropriate variant is written after
sources are copied to multilib builds because of that it is not
propagated to system it self.

Moving it before multilib_copy_source is enough to have fully working
slotted Lua.

Bug: https://bugs.gentoo.org/672528
Signed-off-by: Karel Kočí <cynerd@email.cz>
(cherry picked from commit f08c38554eaeea3a73908bb0f2af6055e3a19b44)
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Fixes: https://github.com/gentoo/gentoo/pull/10571
Fixes: https://bugs.gentoo.org/672528