Un-deprecate rust-toolchain-version or alternative?

By default `dist` will use the GHA runner's default cargo version, which may be a version or two behind the latest stable. Using `rust-toolchain-version` will add a step that installs and forces the provided rust toolchain version. However it is deprecated, asking users to use the `rust-toolchain.toml` file.

My problem is that I develop locally using the nightly toolchain, and would like to release binaries using stable. I don't want developers to be forced to use a stable rust-toolchain if they use nightly locally like I do, because it usually just breaks any command if for example ~/.cargo/config.toml contains nightly-only configs.

Ideally I can keep using rust-toolchain-version, or alternatively I'd inject a single step like `rustup update stable && rustup default stable` (if this is not already possible). What do you think? Thanks!

Start by tracing how cargo-dist handles the deprecated rust-toolchain-version option and the rust-toolchain.toml file on the GitHub Actions runner. Compare preserving the option with supporting a single stable-toolchain setup step, and consider the nightly local-development case; done means a documented, workable release configuration that does not force local developers onto stable.

Written by the indexing model from the issue text.