build-system/go: Fix building on host's secondary architecture.

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.

guix/build-system/go.scm

@@ -163,8 +163,8 @@ commit hash and its date rather than a proper release tag."
                    (tests? #t)
                    (allow-go-reference? #f)
                    (system (%current-system))
-                   (goarch (first (go-target (%current-system))))
-                   (goos (last (go-target (%current-system))))
+                   (goarch #f)
+                   (goos #f)
                    (guile #f)
                    (imported-modules %go-build-system-modules)
                    (modules '((guix build go-build-system)

guix/build/go-build-system.scm

@@ -152,8 +152,10 @@ dependencies, so it should be self-contained."
 
   ;; Make sure we're building for the correct architecture and OS targets
   ;; that Guix targets.
-  (setenv "GOARCH" goarch)
-  (setenv "GOOS" goos)
+  (setenv "GOARCH" (or goarch
+                       (getenv "GOHOSTARCH")))
+  (setenv "GOOS" (or goos
+                     (getenv "GOHOSTOS")))
   (match goarch
     ("arm"
      (setenv "GOARM" "7"))