discovery: properly set the isRemote field for validated networkMsg

This wasn't set properly, leading to some test failures after the prior
change.

discovery/gossiper_test.go

@@ -824,6 +824,8 @@ func TestProcessAnnouncement(t *testing.T) {
 	require.NoError(t, err, "can't create context")
 
 	assertSenderExistence := func(sender *btcec.PublicKey, msg msgWithSenders) {
+		t.Helper()
+
 		if _, ok := msg.senders[route.NewVertex(sender)]; !ok {
 			t.Fatalf("sender=%x not present in %v",
 				sender.SerializeCompressed(), spew.Sdump(msg))