Capture fs locally before setting _ENV=nil in json.lua

sys/modules/opus/json.lua

@@ -39,8 +39,8 @@ if register_global_module_table then
 	_G[global_module_name] = json
 end
 
--- this was incompatible because we use fs later
---local _ENV = nil -- blocking globals in Lua 5.2
+local fs = fs
+local _ENV = nil -- blocking globals in Lua 5.2
 
 pcall (function()
 	-- Enable access to blocked metatables.