Fix sha256sums ordering

Summary:
This fixes the ordering of the files so that they're in alphabetical
order by platform.

Depends on D8370

Test Plan:

./make-sha256sums.sh ~/path/to/gitian-results/0.22.6/ ABC

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8371