[Bugfix] lock Cargo caches during multi-arch builds

[pallasathena92]

What this PR does

• Configures the Cargo registry and Git cache mounts with sharing=locked.
• Prevents concurrent amd64 and arm64 builds from modifying the same Cargo cache.
• Preserves dependency caching while avoiding corrupted crate and xet-core checkouts.

Why we need it

The model-agent image builds both platforms concurrently. BuildKit cache mounts default to shared access, allowing both Cargo processes to modify the same registry and Git checkout.

This intermittently caused missing files such as:

failed to stat '/root/.cargo/git/checkouts/xet-core-.../git_xet/src/git_repo.rs'

Related failure: https://github.com/ome-projects/ome/actions/runs/29587912083/job/87909374980

How to test

Checklist

• Tests added/updated (if applicable)
• Docs updated (if applicable)
• make test passes locally