Put out symlinks in env dir

[Chris Warburton]

asv's other plugins, like virtualenv, use an "env" dir to store their
environments. Ours are automatically kept in the Nix store, which is
nice, but we should probably also put the "out links" from nix-build in
this env dir, so they act as garbage collector roots.

At the moment we're using "--no-out-link", which at least prevents a
buildup of "result.X" links. We should just use the same naming
convention as virtualenv and friends, i.e. using the environment's
"name" attribute.