Make CI compatible with rustup 1.28

[rukai]

As per the announcement: https://blog.rust-lang.org/2025/03/02/Rustup-1.28.0.html#whats-new-in-rustup-1280
There is a breaking change in rustup 1.28 and we must now manually run rustup toolchain install to replace the default toolchain with the toolchain specified in toolchain.toml

The announcement provides a way to remain compatible with both.
But I'll just wait for rustup 1.28 to arrive in github actions images to avoid the compatibility hack, since in the longterm its easier to understand.

[codspeed-hq]

CodSpeed Performance Report

Merging #1875 will not alter performance

_{Comparing rukai:toolchain_install_for_rustup_1.28 (274d8ba) with main (9c19111)}

Summary

✅ 38 untouched benchmarks