Renaming asv setting build_cache_size
The CI job that is checking that the benchmarks don't break is reporting that an `asv` setting has been renamed.

I think we've got a server where we run the benchmarks, not sure if we replicate the environment there at every run from `environment.yml`. Otherwise I guess we should do it manually before merging this.

@TomAugspurger I think you manage that server? does this make sense?
