[guix] Build the OSX DMG

Summary: Generate the OSX DGM image via GUIX. After this GUIX achieves feature parity with Gitian.

Test Plan:

HOSTS=x86_64-apple-darwin ./contrib/guix/guix-build

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15603