powertop module: add kmod to path

powertop attempt to load some kernel modules like msr by calling
modprobe. This is the counterpart to
88e43eb39b which has the powertop
executable search PATH for modprobe rather than hardcoding /sbin, and
actually adds the directory containing modprobe to its PATH for the
systemd service.

nixos/modules/tasks/powertop.nix

@@ -16,6 +16,7 @@ in {
       powertop = {
         wantedBy = [ "multi-user.target" ];
         description = "Powertop tunings";
+        path = [ pkgs.kmod ];
         serviceConfig = {
           Type = "oneshot";
           RemainAfterExit = "yes";