This follows up on 207cf6302 from last year to clean up cases that
have apparently popped up since then. Invoking the scripts directly
makes a cleaner command line in any case, and moreover is essential
to how we control running a Zulip install as either Python 2 or 3
(soon, how we always ensure it runs as Python 3.)
One exception: we're currently forcing `provision` in dev to run
Python 3, while still running both Python 2 and Python 3 jobs in CI.
We use a non-shebang invocation to do the forcing of Python 3.