[guix] Better SHA256SUMS format

Summary: This formats the output hashes the way we deploy it for the release, which makes it easier and less error-prone.

Test Plan: Run the GUIX builds and check the formatting.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16115