Use mktemp for temporary download directory

Using the current directory may not always be appropriate, for example in
the case where it will unnecessarily trigger a backup to be made.

The only risk with this change is that systems might not have a mktemp.
I am not aware of such a system, but have not tested on Windows. It is
working on a basic Ubuntu and OS X installation.
This probably use `$()` instead of backticks, but it must certainly be quoted.