* Buildkite: Add a test for nix-tools closure size
The limit is a rather generous 500MB.
* nix-tools: Reduce closure size
1. Run justStackExecutables on hpack to remove dependency on ghc and
libraries.
Ideally we would be using the component builder on hpack.
2. Replace nix-prefetch-scripts with nix-prefetch-git. stack.yaml
supports mercurial repositories but I've never seen it used.
* Run the nightly scripts to also update the index-state-hashes
This moves the index-state-hashes to `hackage.nix` as well. Thus
we need to have a least one hackage.nix update in.
* fix import
* Update update-index-state-hashes.nix
Add comments.
Fix for #163 was missing something.
Failure was random because the repo is often already unpacked due to
the daily Hackage and Stackage update process.