[upstream staging] Annotate dtolnay/rust-toolchain SHA pin

[duksh]

Summary

Staging PR for upstream submission to pyca/cryptography. Submit upstream via: pyca/cryptography@main...duksh:cryptography:claude/upstream-toolchain-pin-comment

Every other SHA-pinned action carries a version comment; the dtolnay/rust-toolchain pin lacked one because that repo publishes no tags. Annotates all three uses with the branch and commit date (# master, as of Aug 23, 2025), matching the style used for BoringSSL commit pins.

Testing

• Comment-only change; YAML validated.

🤖 Generated with Claude Code

https://claude.ai/code/session_01DWoMomNAcBM3e8duJrb2do

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[duksh]

Submitted upstream as pyca#15326 — closing this staging PR (validation complete: all 93 runnable jobs green; ppc64le legs unrunnable on forks). Do not delete the branch until the upstream PR resolves.

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