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