HPCC-8721 - Delete workunit temporaries

WorkUnit temporary files were already cleared at end of job
but workunit temporary results were not, and these can be
considerably large, accumulating into many 100's MB in Dali.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #3950 from jakesmith/hpcc-8721

HPCC-8721 - Delete workunit temporaries

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>