chore: migrate flake to nixos 26.05 + nix flake update

Signed-off-by: James McCorrie <james.mccorrie@lowrisc.org>

Author: machshev

flake.lock

@@ -5,7 +5,7 @@
   description = "DVSim development environment";
 
   inputs = {
-    nixpkgs.url = "github:nixos/nixpkgs/nixos-25.05";
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-26.05";
     nixpkgs-unstable.url = "github:nixos/nixpkgs/nixos-unstable";
 
     flake-utils.url = "github:numtide/flake-utils";