nixos/prometheus: fix node exporter systemd collector

The systemd collector needs AF_UNIX to talk to
/var/run/dbus/system_bus_socket, which was broken
with 9fea6d4c85.

This commit allows AF_UNIX when needed.

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

@@ -35,6 +35,8 @@ in
           ${concatMapStringsSep " " (x: "--no-collector." + x) cfg.disabledCollectors} \
           --web.listen-address ${cfg.listenAddress}:${toString cfg.port} ${concatStringsSep " " cfg.extraFlags}
       '';
+      # The systemd collector needs AF_UNIX
+      RestrictAddressFamilies = lib.optional (lib.any (x: x == "systemd") cfg.enabledCollectors) "AF_UNIX";
     };
   };
 }