Fix a potential double-prompt in "arc land" when landing with ongoing builds

Summary: The recently-added, build-plan-behavior-aware check here does all of its own prompting, so we should skip the other prompting if it doesn't throw.

Test Plan: Will `arc land` something sketchy sooner or later.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20494

src/workflow/ArcanistLandWorkflow.php

@@ -1376,6 +1376,7 @@ EOTEXT
     // if this one doesn't work out.
     try {
       $this->checkForBuildablesWithPlanBehaviors($diff_phid);
+      return;
     } catch (ArcanistUserAbortException $abort_ex) {
       throw $abort_ex;
     } catch (Exception $ex) {