cmd/go: don't set default GOPATH to GOROOT

It will just cause confusion later as the go tool will say
"warning: GOPATH set to GOROOT (%s) has no effect".
Better to just leave GOPATH unset and get that warning instead.

Change-Id: I78ff9e87fdf4bb0460f4f6d6ee76e1becaa3e7b0
Reviewed-on: https://go-review.googlesource.com/33105
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/go/build/build.go

@@ -264,7 +264,13 @@ func defaultGOPATH() string {
 		env = "home"
 	}
 	if home := os.Getenv(env); home != "" {
-		return filepath.Join(home, "go")
+		def := filepath.Join(home, "go")
+		if def == runtime.GOROOT() {
+			// Don't set the default GOPATH to GOROOT,
+			// as that will trigger warnings from the go tool.
+			return ""
+		}
+		return def
 	}
 	return ""
 }