guix: use proper compiler in cmake toolchain

Summary:
For some reason the -DCMAKE_BUILD_WITH_INSTALL_RPATH=ON is required on the first cmake command to avoid error messages.

Depends on D14065

Test Plan:
contrib/guix/guix-build

Check that it now use the correct cross-compiler to compile Bitcoin ABC.

This is now almost working, down to one wrong symbol (pthread_sigmask) found by symbol-check in bitcoin-qt

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14064