tests/acceptance/ppc_prep_40p.py: NetBSD 7.1.2 location update

tests/acceptance/ppc_prep_40p.py: NetBSD 7.1.2 location update

The NetBSD-7.1.2-prep.iso is no longer available on the CDN, but it's
still available in the archive.

Let's update its location so that users without the file on cache can
still fetch it and run the test.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210924185506.2542588-15-crosa@redhat.com>