nixos: fixup build of aarch64 minimal ISO (fixes #109252)

Perhaps it's not pretty nor precise; feel free to improve.

nixos/modules/tasks/filesystems/ext.nix

@@ -1,4 +1,4 @@
-{ pkgs, ... }:
+{ pkgs, lib, ... }:
 
 {
   config = {
@@ -6,7 +6,10 @@
     system.fsPackages = [ pkgs.e2fsprogs ];
 
     # As of kernel 4.3, there is no separate ext3 driver (they're also handled by ext4.ko)
-    boot.initrd.availableKernelModules = [ "ext2" "ext4" ];
+    # No ext* modules are present on our aarch64, apparently (and maybe elsewhere).
+    boot.initrd.availableKernelModules =
+      lib.optionals (!pkgs.stdenv.hostPlatform.isAarch64)
+        [ "ext2" "ext4" ];
 
     boot.initrd.extraUtilsCommands =
       ''