Back when specific control of the Rust version used with RIOT was
needed, CARGO_CHANNEL was added to explicitly set the Rust version with
consideration for CI special cases.

Rust's mechansims of selecting a toolchain can be used instead now.

The checks would print their prominently colored warnings and confuse
users, and the comments are more geared to those editing the Makefiles
than to those watching which commands get executed.

Contributes-To: RIOT-OS#20088

This pulls in embedded-nal 0.7 implementations, and (on both crates)
alterations that decouple riot-wrappers from riot-sys and simplify build
system integration, and do away with the need for passing
RIOT_USEMODULE.

The latest versions of riot-wrappers and riot-sys do not require it any
more, and the exported data from info-rust is thus much more stable.