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,
@@ -10,6 +11,7 @@
   hostPlatform,
   targetSystem,
   diskoScript,
+  closureExport,
   ...
 }:
 {
@@ -21,8 +23,12 @@
     targetSystem
   ];
 
-  # Explicitly include the system closure in the ISO image to prevent any need to build or fetch
+  # Explicitly include the pre-built closure export and system in the ISO image
   isoImage.contents = [
+    {
+      source = closureExport;
+      target = "/closure-export";
+    }
     {
       source = targetSystem;
       target = "/system";
@@ -53,14 +59,13 @@
       echo ">>> Running disko script..."
       ${diskoScript}
 
-      echo ">>> Copying system closure to target..."
-      # Copy the entire system closure without substitution
-      nix-store --store /mnt --load-db < <(nix-store --dump-db)
-      nix-copy-closure --to /mnt ${targetSystem} 2>/dev/null || true
+      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..."
-      # Install with pre-built closure already in store
-      nixos-install \
+      # Install with pre-built closure already imported (no evaluation or fetching needed)
+      ${pkgs.nix}/bin/nixos-install \
         --no-root-passwd \
         --root /mnt \
         --system ${targetSystem} \