HPCC-10118 Backupnode creating unknown dir in hpcc-mirror, copy of thor

This change will at least reduce the chances of the bug happening. Not sure
it's a complete fix though.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4905 from richardkchapman/backupnode-directories

HPCC-10118 Backupnode creating unknown dir in hpcc-mirror, copy of thor

Merged-by: Gavin Halliday <ghalliday@hpccsystems.com>