Separate eval store for faster remote Nix builds

A pull request landed recently in Nix that is a significant improvement for BioNix and remote building more generally. The problem addressed is that if a remote store is used (say because compute resources are significantly greater elsewhere, or because data can't be transferred for size or privacy reasons) it is also used to evaluate derivations, requiring a significant amount of time due to slow round-trips for each derivation. With the new flag a local store can be used to evaluate the derivations, and the remote store used for builds.

--eval-store and faster closure copying