Revert "nixos/kernel.nix: ensure same kernel is used"

This reverts commit 78f915a002.

nixos/modules/system/boot/kernel.nix

@@ -37,10 +37,12 @@ in
     boot.kernelPackages = mkOption {
       default = pkgs.linuxPackages;
       type = types.unspecified // { merge = mergeEqualOption; };
-      apply = kernelPackages: pkgs.linuxPackagesFor (kernelPackages.kernel.override {
-        inherit randstructSeed;
-        kernelPatches = kernelPackages.kernel.kernelPatches ++ kernelPatches;
-        features = lib.recursiveUpdate kernelPackages.kernel.features features;
+      apply = kernelPackages: kernelPackages.extend (self: super: {
+        kernel = super.kernel.override {
+          inherit randstructSeed;
+          kernelPatches = super.kernel.kernelPatches ++ kernelPatches;
+          features = lib.recursiveUpdate super.kernel.features features;
+        };
       });
       # We don't want to evaluate all of linuxPackages for the manual
       # - some of it might not even evaluate correctly.