Bugfix: do not manually set new OpenLoops and loader paths for all Powheg processes, only ttbb

In order to get the ttbb process to run @tpelzer had implemented a manual setting of some paths via environment variables. However, this was done in the body of a module that gets imported for ALL PowhegControl processes and was thus executed for all of them. This is not the desired behaviour. The present MR encapsulates the behaviour in a function and ensures that it's only used when the ttbb process is run.