Pulls the services.openthread-border-router NixOS module directly from
nixpkgs-unstable since it isn't in 25.11 yet. Service stays disabled
in this commit; configuration follows.
Also promotes `self` from `_module.args` to `specialArgs` in
machines/configuration.nix, since `imports` are evaluated before
`config` and so can't reach `_module.args.self`.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
marthsincemelee
finn.markwitz