use nixbuild.net remote-store building style

[cdepillabout]

nixbuild.net provides two styles of building:

1. remote builder (the "normal" style of remote building with Nix)
2. remote store (a newer feature that is faster in some cases)

This blog post explains the differences. There is also a longer explanation in the nixbuild.net documentation.

We're currently using the remote builder style, but I think the remote store style would work out much better for us, since we need to upload less to nixbuild.net.

This issue is to change our Haskell server to using the remote store style when sending builds to nixbuild.net. One difficulty here is that the Haskell server will need to be smart enough to only use the remote store style when running in AWS, but otherwise just build normally when running in a vm or running locally.

Part of this change will require the following: after a remote-store-enabled nix build is finished, the Haskell server must explicitly nix copy the built output to the machine it is running on in order to pull out the statically-linked binary.