how to override default shell

The `shell` function doesn't seem to respect the `$SHELL` env var or my current users default shell, it always runs `/bin/sh`. Fox example, running:

```haskell
#! /usr/bin/env nix-shell
#! nix-shell -i runghc -p "ghc.withPackages (x: [ x.turtle ])"

{-# LANGUAGE OverloadedStrings #-}

import Turtle

main = do
shell "echo $SHELL" empty
shell "dscl . -read ~/ UserShell" empty
shell "echo $0" empty
```

prints the following:
```
/nix/store/7b6xw0kh049s12j2sqbhdv3n6ma11c91-bash-interactive-4.4-p23/bin/bash
UserShell: /usr/local/bin/fish
/bin/sh
```

Do you know how the underlying `System.Process.shell` determines which shell to use?

Start with Turtle's `shell` function and inspect how it delegates to `System.Process.shell`, using the Haskell reproducer in the issue to compare `$SHELL`, the user's configured shell, and the executed shell. Done means establishing which shell is selected and identifying the supported way to override it, or documenting that limitation.

Written by the indexing model from the issue text.