fix(nix): restrict flake to x86_64-linux (nixpkgs 26.11 dropped darwin)

flake.nix

@@ -11,7 +11,7 @@
   };
 
   outputs = { self, nixpkgs, flake-utils, rust-overlay }:
-    flake-utils.lib.eachDefaultSystem (system:
+    flake-utils.lib.eachSystem [ "x86_64-linux" ] (system:
       let
         overlays = [ (import rust-overlay) ];
         pkgs = import nixpkgs { inherit system overlays; };