I'm trying to get started with a simple [Hello World derivation](https://nixos.org/nix/manual/#ch-simple-expression) using the Nix manual.
But it's not clear to me how to go about building it.

* Is there somewhere I can download the source files from so I don't
have to copy them line-by-line?
* Is there a way I can `nix-build` it
*without* having to modify anything global (eg `pkgs/top-level/all-packages.nix`)?
* Where is `pkgs/top-level/all-packages.nix`?