make-options-doc: fix string context issues

When using `documentation.nixos.includeAllModules = true;` with external
modules, the string context might contain dependencies to derivations
and so `toFile` refuses to evaluate;

```
error: in 'toFile': the file 'options.xml' cannot refer to derivation outputs, at
[...]/nixpkgs/nixos/lib/make-options-doc/default.nix:89:16
```

This is not an issue when using `writeText` (instead of manually
stripping the context).

nixos/lib/make-options-doc/default.nix

@@ -86,7 +86,7 @@ let
   optionsList = lib.sort optionLess optionsListDesc;
 
   # Convert the list of options into an XML file.
-  optionsXML = builtins.toFile "options.xml" (builtins.toXML optionsList);
+  optionsXML = pkgs.writeText "options.xml" (builtins.toXML optionsList);
 
   optionsNix = builtins.listToAttrs (map (o: { name = o.name; value = removeAttrs o ["name" "visible" "internal"]; }) optionsList);