systembus-notify: fix dbus connection failure

      Type = "exec";

      ExecStart = "@out@/bin/systembus-notify";

      PrivateTmp = true;

      ProtectHome = true;

      # NB. We cannot `ProtectHome`, or it would block session dbus access.

      InaccessiblePaths = "/home";

      ReadOnlyPaths = "/run/user";

      ProtectSystem = "strict";

      Restart = "on-failure";

      Slice = "background.slice";