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