unit_carrier_drones.lua: implement drone nanoframe building on carrier.

This should make drone more vulnerable initially.

All drone is currently currently set to 3 second building in drone_defs.lua. It should be changed.

TODO: sea-carrier's drone will build inside sea-carrier's hitbox, this shielded them for attack (this is not an issue for funnelweb). A customization ability for drone-model & colvol offset will be added later for sea-carrier. (if not needed is better! :))