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`]
Note: taking the User Preference `USER_DUP_ACT` into account here (for
both material and world actions) could be a followup step.
Maniphest Tasks: T81248
Differential Revision: https://developer.blender.org/D9046
f79b4850fb