Fix for correct context when adding weapons to player ship

[phkb]

Adding a weapon to the player ship via a script passes a "purchase" context through to the "allowAwardEquipment" condition script call, even when the weapon is added via script. This code change enforces the "scripted" context when a weapon change is made by script.

[phkb]

Belatedly realised that this is what dybal was seeking to achieve with #377. No compile errors on this one, though.