Remove self-hosted subsitution services from lindberg-build

nixos-configurations/lindberg-build/applications/default.nix

@@ -1,4 +1,9 @@
-{ config, pkgs, ... }:
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}:
 {
 
   imports = [
@@ -8,4 +13,11 @@
 
   qois.git-ci-runner.enable = true;
   qois.postgresql.package = pkgs.postgresql_15;
+
+  # Remove substituters that are hosted on this node, to prevent lockups.
+  # The qois-infrastructure cache is not needed,
+  #  since the builds are done (and cached) on this host anyway.
+  nix.settings.substituters = lib.mkForce [
+    "https://cache.nixos.org?priority=40"
+  ];
 }