runtime: clean redundant call of environ function

Change-Id: Ica7179d225c1fb79381f82f58ea5050ac6418b9c
Reviewed-on: https://go-review.googlesource.com/42493Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/env_posix.go

@@ -13,7 +13,7 @@ func gogetenv(key string) string {
 	if env == nil {
 		throw("getenv before env init")
 	}
-	for _, s := range environ() {
+	for _, s := range env {
 		if len(s) > len(key) && s[len(key)] == '=' && s[:len(key)] == key {
 			return s[len(key)+1:]
 		}