[builder] `expandFeaturesToInstances` not working

In https://github.com/googlefonts/gftools/issues/558, it was reported that features weren’t expanding into instances, which meant that feature includes weren’t working. This seems to have been closed by https://github.com/googlefonts/gftools/pull/617.

However, I’m now experiencing the original issue – even when I add `expandFeaturesToInstances: true` to my config.

If I dig into the `instance_ufos` folder, then into a JSON UFO, it starts with this:

```json
{"features":"include(fea/Familyname.fea);"
```

...where I would expect to see the full features, expanded, rather than the original `include()` statement.

The build crashes with the error, when attempting to build static OTFs and TTFs:

```
fontmake: Error: Compiling UFO failed: :1:8: The following feature file should be included but cannot be found: fea/Familyname.fea
```

However, if I do run fontmake directly on the same designspace, with the arg `--expand-features-to-instances`, it works just fine.

Am I missing something, or could this be a regression?