nixos/k3s: use writableTmpDirAsHomeHook when pulling Helm charts (#440157)

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

···
         nativeBuildInputs = with pkgs; [
           kubernetes-helm
           cacert
+          # Helm requires HOME to refer to a writable dir
+          writableTmpDirAsHomeHook
         ];
       }
       ''