HPCC-9186 Roxie cores writing dataset to workunit

A regression introduced when json output support was added, by the looks of
it.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4296 from richardkchapman/roxie-core-wu

HPCC-9186 Roxie cores writing dataset to workunit

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>