Add a script to generate sha256sums from Gitian output

Summary:
Currently, the sha256sums files are assembled by hand (copy-pasting the hash results from the Gitian build).
This patch introduces make-sha256sums.sh to do this automatically.

Test Plan:

make-sha256sums.sh -h
make-sha256sums.sh    # errors as expected
make-sha256sums.sh [path to some gitian output]
make-sha256sums.sh path/to/0.20.9 > jasonbcox-sha256sums.0.20.9

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4893