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.
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.
Examples have previously relied on the (really: some) nightly toolchain
to be the default. As that, in practice, is a problematic assumption,
the latest toolchain to use is now determined programmatically, and that
is set explicitly on the examples that use nightly.
Workaround-For: https://github.com/rust-lang/rustup/issues/3015
