Bug 22435: Fix _set_found_trigger

Bug 22435: Fix _set_found_trigger

The _set_found_trigger utilised the 'Writeoff' offset type to
distinguish between the application of a writeoff and any other form of
offset application.

This patch updates the trigger to use the full link through from offset
to account credit line to get the credit type being offset.

Test plan
1/ Run t/db_dependent/Koha/Items.t and prove it fails before the patch,
but passes after applying this patch.
2/ Run t/db_dependent/Koha/Account/Offsets.t and prove it passes both
before and after applying this patch.

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>