Add nix to shell-stack.nix: allows stack test to run benchmarks
Add nix to shell-stack.nix: allows stack test to run benchmarks

ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell