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.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20494