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
