fix: migrate archived actions-rs to dtolnay/rust-toolchain

[talalashraf]

GitHub removes Node ≤20 action runtimes from runners on 2026-09-16 — changelog. actions-rs/* is archived and stuck on node12, so it's replaced:

• actions-rs/toolchain@v1 (node12, archived) → dtolnay/rust-toolchain

References

• https://linear.app/0xproject/issue/PLA-1666/upgrade-all-node-20-github-actions-org-wide-before-runner-removal

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