Move benchmarks/env.nix to benchmarks/default.nix

[Chris Warburton]

Minor change, but would make things a bit nicer. A couple of reasons we
haven't done this yet:

 - It would alter the sha256 of asv.conf.json, causing a mass
   re-benchmark. That's not too bad on its own.
 - We're pushing benchmark pages with `copy_benchmarks`, which assumes
   there's only a single benchmark cache for each project (as defined by
   the asv.conf.json project name).

We should probably handle this better in `copy_benchmarks` before making
this change. For example, if we made `copy_benchmarks` loop through all
of the caches *first*, accumulating a string of project names, we could
add a simple check to spot if a project has multiple caches, and we can
skip that project with a warning message telling us what the caches are.

That would alert us to the problem, which we could fix manually (by
deleting the older one).