fix(boxes): serialize workspace builds

[AztecBot]

Summary

• Investigated the merge-queue dequeue for feat: merge-train/spartan-v5 #24148 on merge-train/spartan-v5.
• The failing merge-group CI job was 82016013672 / run 27723997361, failing in a1-fast arm64 ci-fast while building boxes.
• The nested boxes build showed boxes/react and boxes/vite concurrently deriving VKs for the same boxreact-BoxReact.json; React's bb child exited with code null while Vite succeeded.
• Serialize the boxes workspace builds by removing -p from yarn workspaces foreach, avoiding concurrent writes to shared VK/cache paths.

Verification

• ./bootstrap.sh ci was attempted, but this branch's root bootstrap script has no ci command and exits with Unknown command: ci.
• HTTP_PROXY=http://172.20.14.2:9803 HTTPS_PROXY=http://172.20.14.2:9803 NO_PROXY=172.20.14.2,host.docker.internal,localhost,127.0.0.1 ./bootstrap.sh build boxes passed.
• A local ci-fast rerun is blocked by workspace setup/tooling issues before this target (AGENTS.md symlink checks and missing formatter/import-sorter tooling), unrelated to the boxes build change.

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Created by claudebox · group: slackbot