treewide: change ubus socket path
Since OpenWrt 21.02, there is an idea to run ubusd process under its own user (non-root) and because of that, they moved this file.
Reference: https://git.openwrt.org/?p=project/ubus.git;a=commit;h=13a4438b4ebdf85d301999e0a615640ac4c9b0a8