guix: use GCC 13 to build releases
Switch release builds to using GCC 13.3.0: https://gcc.gnu.org/gcc-13/, which landed in Guix in: https://git.savannah.gnu.org/cgit/guix.git/commit/?id=750148ce1ea6c65a7c14424546db0078161f7e17.
Does not solve the cross-arch non-determinism for powerpc64le-linux-gnu builds.