nixos/polkit: use tmpfiles to clean old dirs

These don't need to get cleaned up during activation; that can wait
until systemd-tmpfiles-setup runs.

nixos/modules/security/polkit.nix

@@ -88,11 +88,11 @@ in
       "polkit-agent-helper-1".source = "${pkgs.polkit.out}/lib/polkit-1/polkit-agent-helper-1";
     };
 
-    system.activationScripts.polkit =
-      ''
-        # Probably no more needed, clean up
-        rm -rf /var/lib/{polkit-1,PolicyKit}
-      '';
+    systemd.tmpfiles.rules = [
+      # Probably no more needed, clean up
+      "R /var/lib/polkit-1"
+      "R /var/lib/PolicyKit"
+    ];
 
     users.users.polkituser = {
       description = "PolKit daemon";