fix build failure by providing kernel filename to internal NixOS option

The update from 29a10085f3 caused the
uncompressed kernel to not be installed anymore into the kernel output.

This meant that building the top-level argument failed as a file assumed
to exist wasn't there anymore.

We now provide the appropriate filename when we know it.

modules/initrd-kernel.nix

@@ -103,6 +103,10 @@ in
     else (pkgs.recurseIntoAttrs (pkgs.linuxPackagesFor cfg.package))
   );
 
+  config.system.boot.loader.kernelFile = mkIf (cfg.package != null && cfg.package ? file) (
+    mkDefault cfg.package.file
+  );
+
   # Disable kernel config checks as it's EXTREMELY nixpkgs-kernel centric.
   # We're duplicating that good work for the time being.
   config.system.requiredKernelConfig = lib.mkForce [];