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).