darwin: enable linux builder

modules/darwin/nix.nix

@@ -4,6 +4,26 @@
   # auto upgrade nix package and the daemon service.
   services.nix-daemon.enable = true;
 
+  # enable the linux builder
+  nix.linux-builder = {
+    enable = true;
+    package = pkgs.darwin.linux-builder-x86_64;
+    ephemeral = true;
+    maxJobs = 4;
+    config = {
+      virtualisation = {
+        darwin-builder = {
+          diskSize = 40 * 1024;
+          memorySize = 8 * 1024;
+        };
+        cores = 6;
+      };
+    };
+  };
+
+  # enable trusted users
+  nix.settings.trusted-users = [ "@admin" ];
+
   # nix auto garbage collection
   nix.gc = {
     automatic = true;