nixos-module/server/lxc-containers: switch back to using ConditionPathExists

much more graceful to systemd

nix/nixos-module/server/lxc-containers.nix

@@ -176,22 +176,10 @@ in
   systemd.services."lxc@" = {
     description = "LXC container '%i'";
     after = [ "network.target" ];
+    unitConfig.ConditionPathExists = "/var/lib/lxc/%i/rootfs/init";
     serviceConfig = {
       Type = "simple";
-      ExecStart =
-        let
-          script = pkgs.writeScript "start-lxc-container.sh" ''
-            #! ${pkgs.runtimeShell} -e
-
-            if [ ! -e /var/lib/lxc/$1/rootfs ]; then
-              echo "Container $1 does not exist. Run: build-container $1"
-              exit 1
-            fi
-
-            exec ${pkgs.lxc}/bin/lxc-start -F -C -n $1
-          '';
-        in
-          "${script} %i";
+      ExecStart = "${pkgs.lxc}/bin/lxc-start -F -C -n %i";
       ExecStop = "${pkgs.lxc}/bin/lxc-stop -n %i";
       ExecReload =
         let