fix: switch to nixpkgs 25.11 stable to match pre-built uConsole kernel

The uConsole CM5 has kernel 6.12.47 in its store from the
nixos-uconsole-cm5-v1.1.0 image (built with nixpkgs 25.11).
By switching to the same nixpkgs version, Nix will reuse
the existing kernel instead of rebuilding it from source.

flake.nix

@@ -2,7 +2,7 @@
   description = "Gortium infra flake";
 
   inputs = {
-    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+    nixpkgs.url = "github:nixos/nixpkgs?ref=25.11";
     agenix = {
       url = "github:ryantm/agenix";
       inputs.darwin.follows = "";