os: drop unused return value in TestLookupEnv

Change-Id: Ibf227dcfefa179b1c3378476bcd17100b1b1c01e
Reviewed-on: https://go-review.googlesource.com/81375
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/env_test.go

@@ -134,7 +134,7 @@ func TestLookupEnv(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to release smallpox virus")
 	}
-	value, ok = LookupEnv(smallpox)
+	_, ok = LookupEnv(smallpox)
 	if !ok {
 		t.Errorf("smallpox release failed; world remains safe but LookupEnv is broken")
 	}