scylla_util.py: change debug log directory to /var/tmp/scylla
scylla_util.py: change debug log directory to /var/tmp/scylla

Current debug log is bit difficult to collect in CI, to find the debug log
we must know which script caused Exception.
Because the filename does not include prefix, and also specified
directory is shared with other programs.

To make things more easily, let's change debug log directory to /var/tmp/scylla.

Closes #10730

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla