WIP: Add nix backend

[grihabor]

Nix takes the reproducible builds to the next level. This pr introduces nix_binary target that allows to install a binary via nix and use it in pants with pants run.

How to use

1. Install nix
2. Define a nix_binary, for example, dot binary from graphviz nix package:

nix_binary(
    name="dot",
    expr='(import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/06278c77b5d162e62df170fec307e83f1812d94b.tar.gz") {}).graphviz',
    rel_path="bin/dot",
)

3. Create a graph.dot: digraph g { a -> b}
4. Run pants run :dot -- graph.dot

In this example the version of nixpkgs is pinned to a specific commit which guarantees reproducible dot binary.