chore: update flake.lock (#1480)

## What?

Updates `flake.lock` to the latest revisions of all flake inputs
(`nixpkgs`, `flake-utils`, etc.).

## Why?

Keeps Nix inputs current so contributors and CI build against fresh
`nixpkgs`. Picks up upstream security and toolchain fixes. Generated
automatically by the flake-lock update workflow on changes to `go.sum`.

Author: floatpanebot