nixos/k3s: disable unifiedCgroupHierarchy

This gets automatically disabled by docker if the docker backend is
used, but the bundled containerd also doesn't seem to support cgroupsv2,
so disable it explicitly here, too.

nixos/modules/services/cluster/k3s/default.nix

@@ -76,6 +76,10 @@ in
       enable = mkDefault true;
     };
 
+    # TODO: disable this once k3s supports cgroupsv2, either by docker
+    # supporting it, or their bundled containerd
+    systemd.enableUnifiedCgroupHierarchy = false;
+
     systemd.services.k3s = {
       description = "k3s service";
       after = mkIf cfg.docker [ "docker.service" ];