Add prefix argument to make-sha256sums.sh

Summary:
This fixes the script so that we can generate separate sha256sums
for Gitian outputs based on some prefix (ABC or BCHN).

Test Plan:

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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8370