rust: Update to riot-wrappers 0.7.15

[chrysn]

Contribution description

This is a simple cargo update for a newly released riot-wrappers; the only thing non-simple about is is that cstr_core is still held at 0.2.4 awaiting roll-out of the latest CI builders (does no harm, this is just for anyone curious why cargo update would still indicate a change for them).

Testing procedure

• Green CI

Issues/PRs references

This unblocks #16458

[maribu]

ACK, thanks for the quick fix!

[chrysn]

Hmpf, fails-to-build-with-old-nightly again. I had test in place this time to prevent that, but if the riot/riotbuild:latest image are out there with the new nightly already (as they've been for some days), there's little I can do other than running all running changes to riot-wrappers through actual murdock.

I could try to publish a version in-between (which is a slight inconvenience, as Rust for some reasons only allows version numbers of up to 3 components), but given how half the workers already manage, and workers still being affected by mysterious errors, I hope this can resolve itself when the remaining workers (the mobi ones) are upgraded.

[chrysn]

This will need all workers on a recent riotdocker. (Just in case s.o can ping the admins, I'm ofl the PC and thus the chat right now).