fdo#75577: Add and use $helppackaddon variable

It was removed by a previous commit, because the code used the wrong,
non-existing variable $helppackpackaddon.

Change-Id: I00ca05ee07585ffd4fbdc75f881749098e6d5329
Reviewed-on: https://gerrit.libreoffice.org/8662
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>