The step API now supports the same functionality that can_fail_build
provides, so standardize on that argument rather than maintaining a
duplicate mechanism.

Bug: 1274644
Change-Id: Idcc3c0dacbab8d2d786b27ec277713063da6d334
Recipe-Manual-Change: build_limited
Recipe-Manual-Change: infra
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3301439Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Garrett Beaty <gbeaty@google.com>