fix: work on making the installer work offline

fix: ensure system closure is installed for derivations needed by the installer

fix: build closure in build-step instead of on iso

installer/auto-install.nix

@@ -1,6 +1,7 @@
 # This module defines a systemd service that automatically installs NixOS to the disk.
 # It is intended to be used in an installation ISO.
-# It expects `targetSystem` (the closure to install) and `diskoScript` (the partitioning script) to be passed as arguments.
+# It expects `targetSystem` (the closure to install), `diskoScript` (the partitioning script),
+# and `closureExport` (the pre-built NAR archive) to be passed as arguments.
 {
   config,
   lib,
@@ -10,9 +11,11 @@
   hostPlatform,
   targetSystem,
   diskoScript,
+  closureExport,
   ...
 }:
 {
+  # Ensure the entire system closure and all dependencies are included in the ISO
   environment.systemPackages = [
     pkgs.git
     pkgs.bashInteractive
@@ -20,6 +23,18 @@
     targetSystem
   ];
 
+  # Explicitly include the pre-built closure export and system in the ISO image
+  isoImage.contents = [
+    {
+      source = closureExport;
+      target = "/closure-export";
+    }
+    {
+      source = targetSystem;
+      target = "/system";
+    }
+  ];
+
   nixpkgs.hostPlatform = hostPlatform;
 
   systemd.services.auto-install = {
@@ -44,8 +59,17 @@
       echo ">>> Running disko script..."
       ${diskoScript}
 
+      echo ">>> Importing pre-built closure into target store..."
+      # Import the closure that was exported at build time
+      ${pkgs.nix}/bin/nix-store --store /mnt --import < /closure-export/closure.nar > /dev/null
+
       echo ">>> Running nixos-install..."
-      nixos-install --no-root-passwd --system ${targetSystem}
+      # Install with pre-built closure already imported (no evaluation or fetching needed)
+      ${pkgs.nix}/bin/nixos-install \
+        --no-root-passwd \
+        --root /mnt \
+        --system ${targetSystem} \
+        --option substitute false
 
       echo ">>> Done. Rebooting."
       systemctl reboot