[guix] unset EMULATE_FHS when not needed

Summary: Currently we set the var, and it stays set when the next host is reached even if it is *darwin*, which causes the darwin build to fail because the --emulate-fhs option is not compatible with using both gcc-toolchain and clang-toolchain.

Test Plan:
Run the MacOS build after any other build:

HOSTS="x86_64-linux-gnu x86_64-apple-darwin"  contrib/guix/guix-build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15634