Stack on nixos fails when ghc not available in nixpkgs, e.g. ghc802

Please follow the steps below for reporting a bug:

Make sure that you are using the latest release (currently stack-1.9.3).
See the [upgrade instructions](http://docs.haskellstack.org/en/stable/install_and_upgrade/#upgrade) to upgrade.

Please use the following schema for your bug report:

### General summary/comments (optional)
LTS-9.21 requires ghc-8.0.2 which is not available in current nixpkgs.

### Steps to reproduce

```
nix-shell -p stack
stack unpack transformers
cd transformers
cat >stack.yaml <<< "resolver: 9.32"
stack build
```

### Expected
It would be nice if Stack would fall back to installing ghc-8.0.2 as it does on other platforms. If that's too hard, an error message saying something like "resolver 9.21 requires ghc-8.0.2. your does not have a derivation for this version, probably because it's too old. Sorry."

### Actual

```
error: attribute 'ghc802' missing, at (string):1:43
(use '--show-trace' to show detailed location information)
```

### Stack version

```
$ stack --version
1.9.3 x86_64 hpack-0.31.1
```

### Method of installation
nix-shell -p stack