Ephemeral environments for Julia+Python packages

I was thinking it would be nice to have something like `uv run --with numpy python script.py` for running things in ephemeral environments. I see my coding agents like using this sort of thing for quickly answering questions.

Perhaps a `juliapkg run` command?

```bash
uv run --with juliacall \
juliapkg run --pkg Statistics --pkg LinearAlgebra python script.py
```

Posting it in this repo rather than juliapkg as the specific home tool is part of the discussion too.