installer: provide app name to post-install as an argument

the post-install tool run by the installer needs to know the path
for the app bundle as the bitmask privilege helper is located  in
the app bundle, earlier it was using a glob pattern "*VPN.app" to
find it, but since in the previous commit the BitmaskVPN name was
dropped and now uses Bitmask instead, this glob pattern no longer
works, so the app name is passed as argument to the tool