fix(apk-polkit-server.in): correct @LIBEXEC_DIR@ path

Otherwise it cannot start

Partially fixes !45 (merged)

This is still not the complete problem. There is still some bug in the new activation process though, since the service does not properly claim the dev.Cogitri.apkPolkit2 name. Starting GS or trying to use gdbus returns "The name dev.Cogitri.apkPolkit2 was not provided by any .service files", or "The name dev.Cogitri.apkPolkit2 does not have an owner". Weirdly enough, re-creating the old service activation, makes it possible to call gdbus and use GS successfully. However, in that circumstance, there ends up being multiple instances of apk-polkit-server running, one from openrc, one from dbus activation. This shows that there's some problem in the name request process, since it should never be allowed to be more than one server listening on dev.Cogitri.apkPolkit2. This can also be seen starting the server process on two alternative terminals.