graph/db: refactor buildNode to not take a pointer

Since the type returned from the DB is not a pointer. This will be
useful later on.

graph/db/sql_migration.go

@@ -192,7 +192,7 @@ func migrateNodes(ctx context.Context, kvBackend kvdb.Backend,
 				pub, id, dbNode.ID)
 		}
 
-		migratedNode, err := buildNode(ctx, sqlDB, &dbNode)
+		migratedNode, err := buildNode(ctx, sqlDB, dbNode)
 		if err != nil {
 			return fmt.Errorf("could not build migrated node "+
 				"from dbNode(db id: %d, node pub: %x): %w",