nixos/lib/make-{single,multi}-disk-zfs-image: fix references to kernel modules

Closes #455144

Previously, image builds with ZFS would fail with

    kernel-modules-shrunk> root module: 9p
    kernel-modules-shrunk> modprobe: FATAL: Module 9p not found in directory /nix/store/r4m7bmihmr40377yh33pn76wp9w33da2-kernel-modules/lib/modules/6.12.53

with said store-path only containing out-of-tree modules.

I'm pretty sure this was just missed to update in
f71277d66d.

nixos/lib/make-multi-disk-zfs-image.nix

@@ -126,6 +126,7 @@ let
     with config.boot;
     [
       kernelPackages.kernel
+      (lib.getOutput "modules" kernelPackages.kernel)
       kernelPackages.${pkgs.zfs.kernelModuleAttribute}
     ]
   );

nixos/lib/make-single-disk-zfs-image.nix

@@ -114,6 +114,7 @@ let
     with config.boot;
     [
       kernelPackages.kernel
+      (lib.getOutput "modules" kernelPackages.kernel)
       kernelPackages.${pkgs.zfs.kernelModuleAttribute}
     ]
   );