[guix] Fix the build for OSX

Summary: This makes it possible to cross build for OSX with GUIX.

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/D15601