Not in it's path, let's try doing it this way then

nixos/hosts/shodan/configuration.nix

@@ -95,6 +95,7 @@
     decky-loader = {
       enable = true;
       package = pkgs.decky-loader-prerelease;
+      extraPackages = [pkgs.python3];
     };
     devices.steamdeck = {
       enable = true;