21.6 powheg: more ttbb updates - AGENE-1670
These change are a bypass of the problem reported in AGENE-1704 concerning OpenLoops libraries in the ttbb(4FS) Powheg-Box-Res process. It just update some environment variables in the python script, which avoid issues when running on some sites. The changes only affect the newly implemented ttbb(4FS) process - see AGENE-1670.