cmd/go: avoid creating new empty environment variables

Broke some tests that assume $GORACE is unset (because it never is).
Those tests are arguably wrong, but this is more robust.

Change-Id: Id56daa160c9e7e01f301c1386791e410bbd5deef
Reviewed-on: https://go-review.googlesource.com/6480
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/main.go

@@ -158,7 +158,9 @@ func main() {
 	// but in practice there might be skew
 	// This makes sure we all agree.
 	for _, env := range mkEnv() {
-		os.Setenv(env.name, env.value)
+		if os.Getenv(env.name) != env.value {
+			os.Setenv(env.name, env.value)
+		}
 	}
 
 	for _, cmd := range commands {