Fetch and check signature and sha256sum of debian ISO instead of comparing against a hardcoded hash

Summary: See title. This is easier to maintain since version bumps only need to modify the first line.

Test Plan: Copy-paste these commands and run them.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4752