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,
@@ -10,6 +11,7 @@
   hostPlatform,
   targetSystem,
   diskoScript,
+  closureExport,
   ...
 }:
 {
@@ -53,14 +55,17 @@
       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..."
-      # Use only local store - don't fetch from binary cache or build
-      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} \
-        --option substitute false \
-        --option build-use-sandbox false
+        --option substitute false
 
       echo ">>> Done. Rebooting."
       systemctl reboot