[guix] Create a print the SHA256SUMS file

Summary: Concatenates all the parts into a single file and print it.

Test Plan:

./contrib/guix/guix-clean && ./contrib/guix/guix-build

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15654