Update flake inputs

1. Use `stdenv.hostPlatform.system` instead of `system`.
2. Use `runCommand` instead of `runCommandNoCC`.
3. Add new nixpkgs instance for neochat.
4. Set niri config to null when other GUI module.

home/david/configurations/Tytonidae/niri.nix

@@ -1,6 +1,9 @@
-{ config, ... }:
+{ config, lib, ... }:
+let
+  cfg = config.david.programs.niri;
+in
 {
-  david.programs.niri = {
+  david.programs.niri = lib.mkIf cfg.enable {
     waybar.settings =
       let
         cfg = config.david.programs.niri.waybar;