Update Debian VM version link to latest

Summary: They like to break these links, so updating it to latest which worked for me.

Test Plan:
Download the iso from the link and verify the sha256sum.
Follow the instructions to build a Debian 9.8.0 VM.

Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2768