Use swapNode for set_node_level and add_node_level

While this is a behaviour change I don't think the old one made any sense.
It's possible that someone hit this before and wrote a workaround for it,
they won't be affected by this change.
It only makes things work that didn't before.

src/script/lua_api/l_env.cpp

@@ -541,7 +541,7 @@ int ModApiEnvMod::l_set_node_level(lua_State *L)
 		level = lua_tonumber(L, 2);
 	MapNode n = env->getMap().getNode(pos);
 	lua_pushnumber(L, n.setLevel(env->getGameDef()->ndef(), level));
-	env->setNode(pos, n);
+	env->swapNode(pos, n);
 	return 1;
 }
 
@@ -558,7 +558,7 @@ int ModApiEnvMod::l_add_node_level(lua_State *L)
 		level = lua_tonumber(L, 2);
 	MapNode n = env->getMap().getNode(pos);
 	lua_pushnumber(L, n.addLevel(env->getGameDef()->ndef(), level));
-	env->setNode(pos, n);
+	env->swapNode(pos, n);
 	return 1;
 }