make high-priority nixbld substituter configurable in netboot image

default.nix

@@ -43,20 +43,20 @@ let
     nixos-install --no-root-password --flake /mnt/etc/nixos#artiq
     ''; 
 
-  customModule = {
+  customModule = prioNixbld: {
     environment.systemPackages = [ autoInstall pkgs.git ];
     documentation.info.enable = false; # https://github.com/NixOS/nixpkgs/issues/124215
     documentation.man.enable = false;
     nix.settings.trusted-public-keys = ["nixbld.m-labs.hk-1:5aSRVA5b320xbNvu30tqxVPXpld73bhtOeH6uAjRyHc="];
-    nix.settings.substituters = ["https://nixbld.m-labs.hk?priority=10"];
+    nix.settings.substituters = [("https://nixbld.m-labs.hk" + (if prioNixbld then "?priority=10" else ""))];
     system.stateVersion = pkgs.lib.trivial.release;
   };
 
 in
-  makeNetboot {
+  { prioNixbld ? false }: makeNetboot {
     modules = [
       <nixpkgs/nixos/modules/installer/netboot/netboot-minimal.nix>
-      customModule
+      (customModule prioNixbld)
     ];
     system = "x86_64-linux";
   }