Before when building for i686-linux on x86_64-linux GOARCH would be set
to amd64, not 386.
* guix/build-system/go.scm (go-build): Set goarch and goos to #f when
not cross-compiling.
* guix/build/go-build-system.scm (setup-go-environment): Set GOARCH
according to the calculated goarch or using GOHOSTARCH and GOOS
according to the calculated goos or using GOHOSTOS.