Update Nix flake and its lockfile

[weirdrock]

Currently pulls in rust 1.83, which is too old to build iamb now.

I'm going to try to keep the nix stuff up to date from now on.

[ulyssa]

Thank you for noticing and fixing this! I'm not familiar enough with Nix to know exactly what the right steps are, but I imagine it should be possible to use https://github.com/cachix/install-nix-action in the GitHub action and run a validate command for Linux and macOS, right?

[weirdrock]

Yeah, it could be set to run nix flake update and bump the nightly version on either a manual dispatch trigger, or a weekly~ timer. I'll look into it tomorrow.

I could also add an action to build the nix package on new PRs, but that might be annoying for contributors.