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>

Conflicts:

	tools/backupnode/backupnode2.cpp

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>