ci: set rust-toolchain inputs explicitly

[decofe]

Summary

• use the pinned dtolnay/rust-toolchain master action implementation for CI steps
• set the intended Rust toolchain explicitly via with.toolchain
• preserve required components/targets such as clippy, rustfmt, miri, and wasm/riscv targets

Testing

• Parsed workflow YAML with PyYAML
• Checked all dtolnay/rust-toolchain steps have a with: block
• Checked old stable/nightly/clippy/miri action SHAs are no longer used

Prompted by: @DaniPopes

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

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_{Comparing centaur/fix-rust-toolchain-inputs-20260629121320 (4c351c8) with main (5921df7)}