Replace RetrieveTaskFromAgent with RetrieveTask

In Arkindex 1.6.2, we will be merging RetrieveTask and RetrieveTaskFromAgent (arkindex/backend#1555 (closed)), so this call to RetrieveTaskFromAgent will need to be changed to RetrieveTask. It will return the same API response as RetrieveTaskFromAgent.

Note that since RetrieveTaskFromAgent disappears entirely, this means workers that are not updated will not be able to run with cache enabled.