testing/clipit: add explicit sysconfdir
otherwise, xdg autostart file gets installed to /usr/etc/xdg/autostart instead (https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html#referencing)
otherwise, xdg autostart file gets installed to /usr/etc/xdg/autostart instead (https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html#referencing)