stack doesn't expose `STACK_YAML` when calling a pure nix-shell

### General summary/comments

In order to test a package against older version of ghc, I keep several `stack-ghc-xyz.yaml` files around and symlink `stack.yaml` to the latest version. I was trying to build an older version using `STACK_YAML=stack-ghc-xyz.yaml stack build` and `stack` would complain that it couldn't find a matching compiler even though I could see `nix` was provisioning the right version.

It turns out that when using the `nix-pure` option, the `STACK_YAML` variable is not passed to the `nix-shell`, so the `stack` running inside `nix-shell` will compare the ghc version it finds with that on the `stack.yaml` and reject it.

### Steps to reproduce

```bash
$ stack new foo; cd foo
$ cat <stack-ghc-84.yaml
resolver: lts-12.20 # ghc 8.4
packages:
- .
EOF
$ cat <stack.yaml
resolver: lts-14.4 # ghc 8.6
packages:
- .
EOF
$ STACK_YAML=stack-ghc-84.yaml stack --nix --nix-pure build
```

### Expected

The project is built using ghc 8.4.4

### Actual

```
$ STACK_YAML=stack-ghc-84.yaml stack --nix --nix-pure build
Getting project config file from STACK_YAML environment
I don't know how to install GHC on your system configuration, please install manually
```

When ran with `--verbose`, we can see that inside the `nix-shell`, `stack` prints:
```
2019-09-07 15:44:32.266849: [debug] Loading project config file stack.yaml
```

### Workaround

Passing `--stack-yaml` works as expected.

### Stack version

```
$ stack --version
2.1.3.1 x86_64 hpack-0.32.0
```

Installed from `nixpkgs`