nixos/prometheus-node-exporter: fix systemd unit for systemd 242 (#63540)

Avoid having a backslash at the end of ExecStart=.

See https://github.com/NixOS/nixpkgs/issues/63533 for details
about the change to systemd's unit parser.

Fixes #63383.

nixos/modules/services/monitoring/prometheus/exporters/node.nix

@@ -32,8 +32,7 @@ in
         ${pkgs.prometheus-node-exporter}/bin/node_exporter \
           ${concatMapStringsSep " " (x: "--collector." + x) cfg.enabledCollectors} \
           ${concatMapStringsSep " " (x: "--no-collector." + x) cfg.disabledCollectors} \
-          --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
-          ${concatStringsSep " \\\n  " cfg.extraFlags}
+          --web.listen-address ${cfg.listenAddress}:${toString cfg.port} ${concatStringsSep " " cfg.extraFlags}
       '';
     };
   };