fix(ci-buildkit): bundle rootlesskit

Andrew Dunnrequested to merge
fix/ci-buildkit-add-rootlesskit into main

POC sim-buildkit-poc surfaced rootlesskit: command not found. moby/buildkit binary release doesn't include rootlesskit (separate project). Adding it.

Renovate tracks via standard ARG pattern. After merge + ci-buildkit:latest rebuild, the POC retries automatically.

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