Send and solve runtime environment variables

It could be nice to have something like:

```python
project.app.environ["HF_HOME"] = "{workspace}/HuggingFace"
project.app.environ["UV_PYTHON_CACHE_DIR"] = "{workspace}/Python"
project.app.environ["HF_HUB_DISABLE_TELEMETRY"] = "1"
...
```

Would need rust to solve for those at runtime in a format! macro 🙂 