discovery: properly set node's color when syncing graph state

This commit fixes an existing bug wherein we would blank out a node’s
color instead of properly setting the field when syncing graph state
with another node This would cause the node to reject the node
announcement and we would generate an we would invalidate the signature
of the node announcement. We fix this simply by properly setting the
node announcement.

discovery/gossiper.go

@@ -251,6 +251,7 @@ func (d *AuthenticatedGossiper) SynchronizeNode(pub *btcec.PublicKey) error {
 			NodeID:    node.PubKey,
 			Alias:     alias,
 			Features:  node.Features.RawFeatureVector,
+			RGBColor:  node.Color,
 		}
 		announceMessages = append(announceMessages, ann)