Packaging: Pivy Stubgen is not enabled on pixi builds

### Area / Workbench affected

None

### Problem description

Pivy uses stubgen to generate .pyi files. This generation is not enabled on pixi CI builds, which also means that release builds don't have them.

### Impact

No .pyi interface files

### Suggested improvement (optional)

_No response_

### References / evidence

https://github.com/FreeCAD/FreeCAD/actions/runs/28652646057/job/84974382269#step:13:472

```
CMake Warning at src/3rdParty/pivy/CMakeLists.txt:194 (message):
stubgen is not available; Pivy .pyi stubs will not be generated
```

### Development version (if relevant)

```shell

```

Start in src/3rdParty/pivy/CMakeLists.txt around the warning at line 194, then inspect the pixi CI build referenced in the issue. Determine why stubgen is unavailable there and verify the pixi CI and release builds produce Pivy .pyi interface files without that warning.

Written by the indexing model from the issue text.