In `main.c:115` the call
`ztimer_sleep(ZTIMER_MSEC, _periodic_ctx.period.ms);` depends on the
module `ztimer_msec`. Apparently, this was previously pulled in
indirectly and the missing dep has gone unnoticed. This adds the
dep explicitly to fix compilation issues that now have surfaced in
the CI.
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.
