Update Rust nightly version

[chrysn]

Closing #1098 would be easiest if we didn't have to cater for the very old nightly. Let's try updating that first.

[kkysen]

Hi! We have an old PR #811 that updated to 1.67, though it's old and needs rebasing. If you'd rather, you could rebase that. That said, I'm gonna fix the issue by just removing git-testament for now, which removes the time dependency entirely. It's nice to have, so if we update the nightly later, we'll add it back, but that should be pretty simple.

[chrysn]

Thanks for the pointer. Yeah, during the build tests I've run into things you've already solved there.

I'll see if I have anything to contribute there (looks active, which is great), and close this one.

If having some outside testing of the git-testament removal helps, I'd be more than happy to run the full RIOT test suite over that PR once it is there. (Or is that just the plan and nobody is working on it? I can give it a jab, especially if that contributes to getting a new released c2rust that can be cargo installed again in the next few days.)

[kkysen]

I'm in the middle of it and I'll put up a PR for it shortly. If you can test it extensively, that'd be great! (Though I don't anticipate problems with time gone). Then we'll make a new release right after.