Avoid duplicating modules in the list of initrd modules

Functionally this doesn't change anything as they get deduplicated
during the initrd build anyway, but when inspecting values in the repl
or running nix-diff, it's kind of ugly to see the same module repeated 4
times.

modules/nixos/boot.nix

@@ -13,16 +13,16 @@ in
     with lib;
     mkIf cfg.enable {
       boot.initrd.availableKernelModules = filter (dm: dm != null) (
-        map
-          (
-            {
-              driver_module ? null,
-              ...
-            }:
-            driver_module
-          )
-          (
-            unique (flatten [
+        unique (
+          map
+            (
+              {
+                driver_module ? null,
+                ...
+              }:
+              driver_module
+            )
+            (flatten [
               # Needed if we want to use the keyboard when things go wrong in the initrd.
               (report.hardware.usb_controller or [ ])
               # A disk might be attached.
@@ -31,7 +31,7 @@ in
               (report.hardware.disk or [ ])
               (report.hardware.storage_controller or [ ])
             ])
-          )
+        )
       );
     };
 }