Modify execution so that output processors' process_job_output() (but
not export_job_output()) is wrapped by the JOB_OUTPUT_PROCESSED signals.
This makes sense conceptually, and is more useful in practice, as there
are already WORKLOAD_RESULT_EXTRACTION and WORKLOAD_OUTPUT_UPDATE
signals sent by the job, if it's desirable to run before output
processors, but after the job results have been collected.