Rust toolchain version should be set only once

[greenhat]

Motivation

Now we have rust toolchain version specified in the rust-toolchain.toml file and in the CARGO_MAKE_TOOLCHAIN env var in ci.yml, which is not ideal. We should set the version only once in rust-toolchain.toml file.

Notes

The newer rust-toolchain.toml format is not supported by actions-rs GA. It only supports the old rust-toolchain format. Plus, the action-rs is not maintained anymore see. So we need to migrate to something else.

[bobbinth]

In other repos (e.g., here) we've moved away from using actions-rs in favor of the methodology used by rust-analyzer.

[bitwalker]

Fixed in #322