Update flake.lock

[A-Walrus]

Nix flake was out-of-date since rust-toolchain was raised to 1.68, but flake.lock was older than that. Ran nix flake update. In the future when raising rust-toolchain version we need to make sure that we update the flake as well

[hannobraun]

Thank you, @A-Walrus, looks good!

In the future when raising rust-toolchain version we need to make sure that we update the flake as well

I don't use Nix, and don't really maintain the Nix flake. But I agree that it would be better to keep the flake up-to-date.

Some thoughts:

• I usually update to the latest stable Rust version pretty soon after it comes out. You and/or other Nix users could track the latest Rust releases and update the flake each time.
• Alternatively, I could update the Nix flake each time I update the Rust version. But there are challenges:
  • I need to remember that I need to do it: Maybe we can make sure of that by adding a check to the CI build.
  • I need to remember what to do: There could maybe be a procedure file (similar to release-procedure.md) that documents how to upgrade the Rust version. (If we added such a file, it probably makes sense to add a procedures/ directory and move the release procedure there too.)
• It would be even nicer if all of that were completely automated.
  • Maybe a command can be added to automator that makes all the necessary changes and creates a pull request.
  • It would be even more convenient, if we had a bot that did that whenever a new Rust version comes out. But I don't know if I wanna deal with hosting that bot somewhere.

Whatever solution we end up with, this is something that a contributor would have to drive (whether that means implementing the automation, or just writing a document that tells me what to do).