Use netbird from nixpkgs stable again

Version in 26.05 is recent enough, so including unstable into the
closure is not necessary anymore.

flake.nix

@@ -88,9 +88,6 @@
 
         specialArgs = {
 	  inherit inputs outputs username;
-          pkgs-unstable = import nixpkgs-unstable {
-            inherit system;
-          };
         };
 
         modules = [

hosts/busch-gpu-docker/default.nix

@@ -1,4 +1,4 @@
-{ inputs, outputs, config, lib, pkgs, pkgs-unstable, ... }:
+{ inputs, outputs, config, lib, pkgs, ... }:
 
 {
   imports =

hosts/busch-main-docker/default.nix

@@ -1,4 +1,4 @@
-{ inputs, outputs, config, lib, pkgs, pkgs-unstable, ... }:
+{ inputs, outputs, config, lib, pkgs, ... }:
 
 {
   imports =

hosts/julius-framework/default.nix

@@ -2,7 +2,7 @@
 # your system.  Help is available in the configuration.nix(5) man page
 # and in the NixOS manual (accessible by running ‘nixos-help’).
 
-{ inputs, outputs, config, pkgs, pkgs-unstable, ... }:
+{ inputs, outputs, config, pkgs, ... }:
 
 {
   imports =
@@ -51,7 +51,6 @@
 
   services.resolved.enable = true;
   services.netbird = {
-    package = pkgs-unstable.netbird;
     useRoutingFeatures = "client";
     clients.wt0 = {
       hardened = true;

modules/netbird-client.nix

@@ -1,6 +1,5 @@
 {
   pkgs,
-  pkgs-unstable,
   utils,
   config,
   lib,
@@ -46,7 +45,6 @@ in {
 
   config = lib.mkIf cfg.enable {
     services.netbird = {
-      package = pkgs-unstable.netbird;
       useRoutingFeatures = "both";
       clients.wt0 = {
         hardened = false;