fix: pin Rust version to 1.73.0 for Sui contracts

[devin-ai-integration]

Fix CI workflow for Sui contracts by pinning Rust version to 1.73.0.

• Add rust-toolchain.toml to specify Rust 1.73.0
• This version is already used successfully in other parts of the project (Near, Solana)
• Fixes time crate compatibility issues in CI

Link to Devin run: https://app.devin.ai/sessions/924c55d986894dd19da6434f80185655
Requested by: Jayant

[devin-ai-integration]

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