[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