nixos: only enable spipe when user specifies

Signed-off-by: Austin Seipp <aseipp@pobox.com>
cryptol v2.0.0

This comes with several extra libraries, including GraphSCC, monadLib,
presburger, process and smtLib, all required as build dependencies. But
otherwise totally automated via cabal2nix.

Next up is CVC4 (a total pain in the ass to package) for proving/SAT
support.

I have another WIP branch for the unfree 1.x series which I may (or may
not) add later as it has external verification tech at the moment.

Signed-off-by: Austin Seipp <aseipp@pobox.com>