Add output for the installed toolchain name

[jschwe]

Would you consider accepting a PR which adds a rust_toolchain_name output which contains the name of the installed toolchain?
E.g. if the input is stable-2, then output.rust_toolchain_name would be 1.64.0. I'm not interested in the version per-se, but more interested in doing cargo +<stable-2> build, but don't want to rely on the installed toolchain being the default toolchain. There are multiple ways to override the default toolchain (e.g. rust-toolchain.toml) and I want to make sure that in my case stable-2 always wins. I could do the version resolution myself, but since the logic is already here in your action, I would prefer if the action would just expose this information.