Re: See if our benchmark setup could be built by Nix

[Chris Warburton]

I've just realised a much more obvious improvement: we provide an
environment via shell.nix for running the benchmarks; this provides asv
and the asv-nix plugin. Why not *also* make it provide all of the
required commands, env vars, etc.?

We currently have three "levels" of Nix builder to run our benchmarks:

 1 nix-shell builds an environment, which we only use for providing asv
   and asv-nix.
 2 benchmarks/default.nix builds an environment, which provides the
   python binary used for invoking the benchmark functions, but isn't
   specific to any particular git commit.
 3 asv-nix provides each particular git commit to the
   benchmarks/default.nix builder, so it can put commit-specific things
   into the environment. It currently ignores that though.

We don't use 3, but it's nice to keep the option open. We currently do
all of the complicated stuff in 2, which gets called *inside* 1, and
hence may run into problems with NIX_PATH and friends.

We should move as much as possible out of 2 and into 1. This way, we
aren't building complicated stuff *inside* a Nix environment, so we can
check it all with release.nix, tests.nix and friends.