Return help when called with --help

Calling with `--help` creates a venv with that name
```bash
$ mkvirtualenv --help
Using CPython 3.14.2
Creating virtual environment with seed packages at: .virtualenvs/--help
 + pip==25.3
Activate with: source .virtualenvs/--help/bin/activate
```

However, invoking `uv venv` does not:
```bash
$ uv venv --help
Create a virtual environment

Usage: uv venv [OPTIONS] [PATH]
[...]
``` 