This can be used to force a particular executable by setting

SIMFACTORY_PYTHONEXECS=path-to-python

Pull request is here: https://bitbucket.org/simfactory/simfactory2/pull-requests/52/sim-let-use-override-pyexe-choices

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2507/let-use-override-pyexe-choices