Transitioning on toolchain channels

[UebelAndre]

I'm looking for a way to have some targets transition on the Rust toolchain channel (stable, nightly, beta) without needing to define a platform with this as a constraint. Defining platforms is difficult since rust_toolchain requires a cc_toolchain and once --platforms (or --host_platform), the host detected cc_toolchain no longer matches. Any thoughts on how a transition could be defined that would allow specific targets to build with a nightly toolchain?

[UebelAndre]

This request is related to #1642 where I think it'd be cool if users weren't forced to define their own platforms or always use a nightly toolchain for the entire workspace just to use the functionality there.

[UebelAndre]

cc @illicitonion @krasimirgg @scentini for ideas