use mktemp instead of $TMPDIR

Running this script on my Ubuntu 14 server gives a permission denied error:
It tries to create a temp dir using $TMPDIR which is not set in my shell, and as I'm not running it as root, it fails to create ```/admin_ui-12322.tmp```.
Using mktemp, which defaults to $TMPDIR if set, else to /tmp, solves it.