Problem you are trying to solve

When running a project, often times, one wants to use nightly features from rustfmt, while also keeping a stable version for builds + deploys.

Solution you'd like

Allow a second key called [[other-toolchain]] to specify non-default toolchains. This should be backwards-compatible with the current rust-toolchain format:

# this is for builds
[toolchain]
channel = "1.70.0"
targets = [ "wasm32-unknown-unknown", "aarch64-linux-android" ]

# this is for rustfmt
[[other-toolchain]]
channel = "nightly-2023-11-26"

Notes

No response