chore: Update Zig to 0.15.2

[sagehane]

I updated the project to Zig 0.15.2 and updated the vastly outdated Nix Flakes files. I didn't set the version to 0.15.1 because 0.15.2 arguably introduces a breaking change.

For the example program, I changed the stdout writer to the new API but kept the stdin reader to the deprecated one, as the semantics of the example depends on it. It should be handled soon though, as the API has been deleted and will be gone by 0.16.0.

For Nix Flakes, I changed it to depend on https://github.com/mitchellh/zig-overlay to simplify the fetching of Zig binaries. This also comes with the added effect that it supports building for Aarch64, but I never tried Tracy on it so idek if that works. Whatever.