Only inject legacy Arcanist workflows into "help" if run from the context of an Arcanist runtime
Summary: Ref T13490. This code is reachable from Phabricator binaries; only inject the legacy stuff if we're in an Arcanist stack.
Test Plan: Ran bin/conduit help from phabricator/.
Maniphest Tasks: T13490
Differential Revision: https://secure.phabricator.com/D21113