fix tox build for environment-python != containing-python in yet-another instance
fix tox build for environment-python != containing-python in yet-another instance

this instance: the repository worktree is *not* named borg.

edgimar/borg,edgewood/borg,raxenak/borg,edgewood/borg,edgimar/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,raxenak/borg,edgewood/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,edgimar/borg,RonnyPfannschmidt/borg,edgimar/borg,raxenak/borg,raxenak/borg,edgewood/borg