`download` script requires pip

I use `uv` without pip, so I get the following:

```
(venv) PS C:\Users\me\> python -m spacy download en_core_web_sm
C:\Users\me\.venv\Scripts\python.exe: No module named pip
```

I don't think you need a script to call pip for users.
If your models are all pip-installable packages, they should `depend` on their supported versions of spacy. Then the user's dependency resolver can find the correct model version to download.