tests/avocado/machine_m68k_nextcube: Fix the download URL for the ROM image

If Avocado has to fetch this asset, the download fails with a 403 HTTP
error. Use a different URL to fix the issue.

Message-ID: <20231101201934.27637-1-huth@tuxfamily.org>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>