Rather than migrating to Bazel, it seems more appropriate to use Nix
for this project.
The project is split into several different components (for data
dependencies and binaries). A derivation for building an image for
Nixery itself will be added.
Initial version of tool provider via Nix. This requires two separate
steps for adding a new tool:
1. New symlink in tools/bin to point at the dispatch script.
2. Mapping of tool to Nix package set attribute in dispatch script.
