import-rdiff-backup-cmd: invoke mktemp more portably

This should fix a problem on NetBSD 6, reported by Greg Troxel
<gdt@lexort.com>.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Reviewed-by: Greg Troxel <gdt@lexort.com>
Tested-by: Greg Troxel <gdt@lexort.com>