tests/functional/test_aarch64_sbsaref_freebsd: Fix the URL of the ISO image

tests/functional/test_aarch64_sbsaref_freebsd: Fix the URL of the ISO image

The original image has been removed from the server, so the test
currently fails if it has to fetch the asset, but we can still
download the ISO from the archive server. While we're at it, prefer
the XZ compressed image, it's much smaller and thus the download
should be faster.

Message-ID: <20250701105809.366180-1-thuth@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>