First, it's leaking information. Second, the URLs can be really long and
cause "OSError: [Errno 36] File name too long".
The same issue has already been fixed in the image cache, this change
updates building ISO. Since the files are created in a temporary
location, just use constant names.
Change-Id: Ie5e0523bb4a8941bcb8dc87c4a0b29cb28ddfe40
2d885126ec