stack2nix 0.2.2 doesn't support stack.yaml using ghc-head

[TerrorJack]

When calling stack2nix with a stack.yaml file including resolver: ghc-8.9 (with extra setup-info fields containing the ghc-head bindist urls), stack2nix fails with:

Ensuring git version is >= 2 ...
Ensuring cabal version is >= 2 ...
stack2nix: haskell.compiler.ghc89 failed to build via nix
CallStack (from HasCallStack):
  error, called at src/Stack2nix/Util.hs:79:22 in stack2nix-0.2.2-5woVHnZZPquEpQd1dyekQ7:Stack2nix.Util

[domenkozar]

stack2nix supports only ghcs in nixpkgs, we should probably mention that.

Try haskell.compiler.ghcHEAD