fix: switch to the latest testing kernel on thonkpad

The new Intel Core Ultra does not work that well with older kernel versions
especially since the initial support was added in 6.8. Currently, the testing
kernel is much more stable that the latest package on this processor.

Signed-off-by: Chinmay D. Pai <chinmaydpai@gmail.com>

systems/x86_64-linux/thonkpad/hardware.nix

@@ -1,4 +1,8 @@
-_: {
+{
+  lib,
+  pkgs,
+  ...
+}: {
   boot = {
     initrd = {
       availableKernelModules = [
@@ -16,6 +20,7 @@ _: {
       "iwlwifi"
       "xe"
     ];
+    kernelPackages = lib.mkDefault pkgs.linuxPackages_testing;
     kernelParams = [
       # NixOS produces many wakeups per second, which is bad for battery life.
       # This disables the timer tick on the last 7 cores.