Respect `tool.uv.environments` in `pip compile --universal` (#6663)
## Summary
We now respect the `environments` field in `uv pip compile --universal`,
e.g.:
```toml
[tool.uv]
environments = ["platform_system == 'Emscripten'"]
```
Closes https://github.com/astral-sh/uv/issues/6641.
