This adds the //nix/lazy-deps tool at //lazy-deps in tvl-kit.
A CI step is added for the kit that uses this to lazily build an
example tool (magrathea).
Change-Id: Ibd6d69c83b87bd6e0766942d73297621f2593113
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5514
Tested-by: BuildkiteCI
Reviewed-by: ezemtsov <eugene.zemtsov@gmail.com>