This was already changed for Material nodetrees on duplication in
rBa75ac18638f4.
Since it is not obvious from the UI how change World actions - and to be
consistent with Material actions, it is best to copy the action as the
default behavior.
So use generic BKE_id_copy functions with LIB_ID_COPY_ACTIONS flag [which
also enables us to get rid of BKE_world_copy]
I would also propose to take the User Preference USER_DUP_ACT into
account (for both material and world actions) as a followup step?