add nix-index

nixos/shell.nix

@@ -67,6 +67,8 @@
   # even though we don't use bash as our shell, xonsh uses bash completion scripts
   programs.bash.completion.enable = true;
 
+  programs.command-not-found.enable = false;
+
   users.defaultUserShell = pkgs.xonsh;
   programs.xonsh =
     let
@@ -82,6 +84,7 @@
         xontrib load clp
         xontrib load direnv
         xontrib load sh
+        source-bash ${pkgs.nix-index}/etc/profile.d/command-not-found.sh
       '';
       package = pkgs.xonsh.override {
         extraPackages = ps: [