Fix hero switch switching the wrong clients

I love git bisect! The problem was due to me accidentally deleting a
crucial condition `is_network_master` when checking for hero_switch
input.

scripts/player.gd

@@ -57,6 +57,7 @@ func spawn():
 	set_linear_velocity(Vector3())
 
 func _input(event):
+	if is_network_master():
 		if Input.is_action_just_pressed("switch_hero"):
 			switch_hero_interface()
 		# Quit the game: