zookeeper: refactor to use finalAttrs
- wrap derivation in `stdenv.mkDerivation (finalAttrs: { ... })`
- replace `${pname}`/`${version}` with `${finalAttrs.version}` in `src` URL
- update `meta.changelog` to interpolate `finalAttrs.version`
This removes reliance on `rec` and ensures attribute references are
scoped through `finalAttrs`, improving clarity and consistency with
current Nixpkgs conventions.