contrib: simplify the genosxversion.py command to find the hg libraries I forget what problem I ran into while trying to teach the makefile to use a non-system python. (It might have ben missing hg-evolve and/or keyring, but `check_output()` was raising an error.) This still isn't great because it will return non zero for something like the username not being set, even though we aren't asking for it. But I suppose it's still useful to simplify. Differential Revision: https://phab.mercurial-scm.org/D6753