Nix integration should use GC roots

Usual scenario:

1. build stack project with Nix
2. sometime later you run nix-store --gc or nix-collect-garbage
3. you run into following error:

/home/user/.stack/setup-exe-cache/x86_64-linux-nix/Cabal-simple_mPHDZzAJ_1.24.2.0_ghc-8.0.2: streamingProcess: runInteractiveProcess: exec: does not exist (No such file or directory)

This means that nix libraries that the file was compiled with are missing from nix store.

Best way to prevent this is to create Nix GC roots. Also stack could double check that those libraries are still intact.