net: enable TestLookupDotsWithRemoteSource on builders

Change-Id: I2609660b10a16ec2a256fc9c8e046ba4ae67963f
Reviewed-on: https://go-review.googlesource.com/18880Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/lookup_test.go

@@ -466,7 +466,7 @@ func TestLookupDotsWithRemoteSource(t *testing.T) {
 	if testing.Short() && testenv.Builder() == "" || !*testExternal {
 		t.Skip("avoid external network")
 	}
-	if !supportsIPv4 || *testIPv4 {
+	if !supportsIPv4 || !*testIPv4 {
 		t.Skip("IPv4 is required")
 	}