guix: use GCC 11 for macOS builds

Summary:

Note that this is just the native compiler, that is used to build the
toolchain we use to build the actual binaries.

Backport of core#29673.

Depends on D17233.

Test Plan: Run the OSX guix build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17234