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