buildLinux: kernel.configEnv: comply with overrideAttrs

Override `finalAttrs.finalPackage` instead of `kernel` from the let-in
block to make configEnv behave in accordance with overrideAttrs of the
result package.

Continuation of commit f10331cf2e ("buildLinux: passthru by
<pkg>.overrideAttrs instead of lib.extendDerivation")

pkgs/os-specific/linux/kernel/generic.nix

@@ -239,8 +239,8 @@ kernel.overrideAttrs (finalAttrs: previousAttrs: {
 
     # Adds dependencies needed to edit the config:
     # nix-shell '<nixpkgs>' -A linux.configEnv --command 'make nconfig'
-    configEnv = kernel.overrideAttrs (old: {
-      nativeBuildInputs = old.nativeBuildInputs or [] ++ (with buildPackages; [
+    configEnv = finalAttrs.finalPackage.overrideAttrs (previousAttrs: {
+      nativeBuildInputs = previousAttrs.nativeBuildInputs or [ ] ++ (with buildPackages; [
         pkg-config ncurses
       ]);
     });