Lua: Don't parallel LuaRocks installations

Some LuaRocks mirrors are limiting the amount of allowed connections, this will
cause the build to fail when make installs these in parallel.

Reported-by: William Dauchy <william@gandi.net>
Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>