Upgrade nixpkgs and monitor nodes via LAN

Apparently it causes the Docker from the gitlab CI to break: https://github.com/NixOS/nixpkgs/issues/245365

Let's see if they fix it, or I can find a workaround.

This MR disables the proxy by default in all nodes, as the ssfhead node is now configured a as a NAT router and forwards the traffic to the Internet.

This also fixes #26 (closed) by disabling the all_proxy variable.