Add all the relevant tools to compile (only `cargo` needed, `rustc`
is bundled with it in the right version) and develop (`clippy` and
`rustfmt` can then directly be called from cargo; `rust-analyzer`
provides a useful language server for editor).
Made it possible to override nixpkgs by providing an overridable
`pkgs`.
Not sure if the `LD_LIBRARY_PATH` is needed; left it like this for
now.
Signed-off-by: Henner Zeller <h.zeller@acm.org>
