dhallToNix: Permit inputs referring to derivations (#134459)
dhallToNix: Permit inputs referring to derivations (#134459)

Fixes https://github.com/dhall-lang/dhall-haskell/issues/2267

`pkgs.dhallToNix` currently fails when a Dhall package is
interpolated into the input source code, like this:

```nix
let
  pkgs = import <nixpkgs> { };

  f = { buildDhallPackage }: buildDhallPackage {
    name = "not";
    code = "λ(x : Bool) → x == False";
    source = true;
  };

  not = pkgs.dhallPackages.callPackage f {};

in
  pkgs.dhallToNix "${not}/source.dhall True"
```

This is because `dhallToNix` was using `builtins.toFile`, which
does not permit inputs with interpolated derivations.  However,
`pkgs.writeText` does not have this limitation, so we can switch
to using that instead.
NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs