Use nixos stable for nix flake (payjoin#1194)

Author: DanGould

flake.lock

@@ -2,7 +2,7 @@
   description = "rust-payjoin";
 
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
     flake-utils.url = "github:numtide/flake-utils";
     rust-overlay = {
       url = "github:oxalica/rust-overlay";