image_builder: Force the creation of mount_dir to be in /tmp

[fidencio]

Description of problem

When tryung to run image_builder.sh on immutable systems, such as Red Hat Core OS and Fedora Core OS, those will not allow mount_dir to be created in a location that's not read-write (and those have a lot of restrictions of which are the read-write places in the distro).

Expected result

Create mount_dir in /tmp, as it's always read-write for all distros.

Actual result

The script fails on such distros, depending where it's called from.