Set UV_PYTHON in Jupyter kernels (astral-sh#11155)

## Summary

It turns out activating the kernel does not change `VIRTUAL_ENV`, so we
still install into the environment the Jupyter environment, rather than
the project environment.

Unfortunately, after this change, we do still show a warning on `uv
add`:

```
warning: `VIRTUAL_ENV=/Users/crmarsh/.cache/uv/archive-v0/3bddKDdYXuX2w57Fu6itL` does not match the project environment path `.venv` and will be ignored
```

`uv pip install` works without warning.

Closes astral-sh#11154.

Author: charliermarsh

docs/guides/integration/jupyter.md

@@ -51,7 +51,7 @@ $ uv add --dev ipykernel
 Then, you can create the kernel for `project` with:
 
 ```console
-$ uv run ipython kernel install --user --name=project
+$ uv run ipython kernel install --user --env VIRTUAL_ENV $(pwd)/.venv --name=project
 ```
 
 From there, start the server with: