HPCC-9293 Ensure spill names are consistent for each compile

It makes it easier to compare locally generated work units, and
the simpler names make the spills easier to read.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6553 from ghalliday/issue9293

HPCC-9293 Ensure spill names are consistent for each compile

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>