fix(nix): use nixos-unstable

Fixed #4947

Author: doprz

flake.lock

@@ -2,7 +2,7 @@
   description = "A simple terminal UI for git commands";
 
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
     systems.url = "github:nix-systems/default";
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-compat.url = "https://flakehub.com/f/edolstra/flake-compat/1.tar.gz";