flake: Bump nixpkgs

Updates nixpkgs flake input. Also switches the input type to the channel
tarballs, since infra now supports the lockable tarball protocol.

Author: xokdvium

flake.lock

@@ -1,7 +1,7 @@
 {
   description = "The purely functional package manager";
 
-  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
+  inputs.nixpkgs.url = "https://channels.nixos.org/nixos-25.05/nixexprs.tar.xz";
 
   inputs.nixpkgs-regression.url = "github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2";
   inputs.nixpkgs-23-11.url = "github:NixOS/nixpkgs/a62e6edd6d5e1fa0329b8653c801147986f8d446";