Fix PXE netboot installer hanging at boot (ISO/netboot module conflict)

The netboot build composed ./modules/installer/iso.nix (which pulls in
nixpkgs' installation-cd-minimal.nix) together with nixpkgs'
netboot-minimal.nix. Both installation-cd-base.nix and netboot.nix set
fileSystems."/" via the identical lib.mkImageMediaOverride (mkOverride
60) priority - genuinely conflicting root-filesystem strategies
(ISO-by-label vs. netboot-tmpfs) at the same priority, and the ISO one
was winning. Every netboot boot hung waiting for a device that can
never exist outside a real CD/USB:

  A start job is running for /dev/disk/by-label/nixos-minimal-...

Reproduced live: deployed a scratch lxc-pxe-boot on pve-test, pulled its
built kernel/initrd, and booted them directly with QEMU to confirm the
hang and capture full console output. netboot-minimal.nix's own chain
(netboot-base.nix) already imports profiles/installation-device.nix
independently, so common.nix's initialHashedPassword override still
applies correctly with iso.nix removed from this composition. Rebuilt
and re-booted the same way after the fix - full boot to a working shell
with SSH up, no hang.

flake.nix

@@ -119,10 +119,24 @@
 
       # Same installer environment, built as netboot (kernel + initrd +
       # iPXE script) instead of an ISO — this is what packages.pxe bundles.
+      #
+      # Deliberately imports common.nix directly, NOT ./modules/installer/iso.nix
+      # (which pulls in nixpkgs' installation-cd-minimal.nix) -- confirmed live
+      # that composing the ISO module together with netboot-minimal.nix hangs
+      # every boot waiting for a device that can never exist on a netboot
+      # client ("A start job is running for /dev/disk/by-label/nixos-minimal-...").
+      # Both installation-cd-base.nix and netboot.nix set fileSystems."/" via
+      # the identical lib.mkImageMediaOverride (mkOverride 60) priority --
+      # genuinely conflicting root-filesystem strategies (ISO-by-label vs.
+      # netboot-tmpfs) at the same priority, and the ISO one was winning.
+      # netboot-minimal.nix's own chain (netboot-base.nix) already imports
+      # profiles/installation-device.nix independently, so common.nix's
+      # initialHashedPassword override (which assumes that profile is
+      # present) still applies correctly without iso.nix in the mix.
       netbootSystem = nixpkgs.lib.nixosSystem {
         inherit system;
         modules = [
-          ./modules/installer/iso.nix
+          ./modules/installer/common.nix
           ({ modulesPath, ... }: {
             imports = [
               (modulesPath + "/installer/netboot/netboot-minimal.nix")