Use the `update-flake.sh` script to auto-update the NixOS version at
various places.
Also update `examples/container/flake.nix` which has been
missing in the update instructions.
Some of the commands listed in usage.sh require clightning to be
enabled. This commit enables clightning so they will work. It also
removes electrs to keep things simple, since electrs is not in
usage.sh