The original code from 56d0d416 only stands a chance at working on RT
3.6.  RT 3.8 and 4.0 use the modern config handling and call
RT->Config->Get('Plugins') to load plugins.  Get() doesn't read values
from the RT:: namespace; it uses a secondary store which we never
affected.  By calling Set() ourselves we ensure the config is properly
updated and seen by the subsequent RT::Init() call.