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
This better reflects the current state of things. Setting the nightly
toolchain as default works better for new users with the current
settings of the examples. Not pinning the C2Rust version -- we don't pin
the nightly version in the docs either, and breakage is easy to track
given the container builds as a base line.
The duplication happened already in the initial version, and when
[17504] had the `--locked` added, the opportunity to remove the
duplication was missed.
[17504]: https://github.com/RIOT-OS/RIOT/pull/17504
The `--locked` is required since dependencies increased their MSRV to
something later than C2Rust's fixed nightly. That was already reflected
in the riotdocker installation steps, but not here where the line was
taken from upstream's docs.
Closes: https://github.com/RIOT-OS/RIOT/issues/17492
