Consider using nixos stable channel #1187

With the benefits of the unstable channel not really giving us any major benefits we should probably use the stable channel for our nix flake. This would also bring us closer to being able to continue to merge the ohttp-relay following the similar flake change. payjoin/ohttp-relay#76

In addition to this change we should also update the flake more regularly to fully get the benefits of this flake in the first place, relates to #1165 as this could be replaced with a flake based CI job