Use buildctl wmf-v0.12.5-2

Use the newer version of buildctl from our updated v0.12 fork. For now,
pull from registry.cloud.releng.team until we're unblocked on building
buildkit with a Dockerfile on trusted runners.

Change use of --wait-for-ready to --wait. The latter is the
refactored form of the accepted upstream contribution. See
https://github.com/moby/buildkit/pull/3586

Bug: T356418