add package for 'biri', a fork of niri with PRs merged

flake.nix

@@ -4,6 +4,8 @@
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
 
+    nixpkgs-26-05.url = "github:nixos/nixpkgs/nixos-26.05";
+
     flake-parts.url = "github:hercules-ci/flake-parts";
 
     import-tree.url = "github:vic/import-tree";