Fix builtin item metatable (#2328)

Co-authored-by: Alexander Weber <web.alexander@web.de>

mods/default/item_entity.lua

@@ -74,5 +74,5 @@ local item = {
 }
 
 -- set defined item as new __builtin:item, with the old one as fallback table
-setmetatable(item, builtin_item)
+setmetatable(item, { __index = builtin_item })
 minetest.register_entity(":__builtin:item", item)