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

src/parser/argument/workflow/PhutilHelpArgumentWorkflow.php

@@ -40,6 +40,7 @@ EOHELP
     $with = $args->getArg('help-with-what');
 
     $runtime = $this->getRuntime();
+    if ($runtime) {
       $toolset = $runtime->getToolset();
       if ($toolset->getToolsetKey() === 'arc') {
         $workflows = $args->getWorkflows();
@@ -77,6 +78,7 @@ EOHELP
 
         $args->setWorkflows($workflows);
       }
+    }
 
     if (!$with) {
       $args->printHelpAndExit();