See if our benchmark setup could be built by Nix

[Chris Warburton]

The setup_cache functions in our benchmarks are getting quite
complicated. There's a lot this that we could probably do with Nix,
rather than just giving access to a few env vars.

We specifically DON'T want to run the benchmarking tools via Nix, since
that can cause them to interfere (e.g. if Nix builds a bunch of stuff in
parallel).

We can do the sampling, translation, etc. up front in Nix.

We can turn the parameters.py file into a JSON file, which can be read
by Nix (and maybe passed in to Python via the environment?)

Analysing the results will still need to be done from Python, since that
requires the stdout of the benchmarked runs. We might be able to package
this up a little better though.

I think there's a balance to be struck in general:

 - Nix is good at running programs, reading files, setting up
   environments, etc. so it should be used for that.
 - Python is good for manipulating data (e.g. JSON or otherwise).

We should focus each on their strengths. In particular, we can use
Python instead of jq in Nix, and we can use Nix instead of subprocess32
in Python.