`pipenv run generate` does not work (outdated instructions?)

### Summary

`pipenv run generate` does not work (outdated instructions?)

### Reproduction Steps

```
pipenv run generate
```

### Expected Results

Data being generated, as indicated in https://github.com/danielgtaylor/python-betterproto/blob/master/tests/README.md#running-the-tests

### Actual Results

```
$ pipenv run generate
Error: the command generate could not be found within PATH or Pipfile's [scripts].
```

### System Information

```
libprotoc 28.0
Python 3.12.6
Name: betterproto
Version: 2.0.0b7
Summary: A better Protobuf / gRPC generator & library
Home-page: https://github.com/danielgtaylor/python-betterproto
Author: Daniel G. Taylor
Author-email: danielgtaylor@gmail.com
License: MIT
Location: /tmp/python-betterproto-v.2.0.0b7/.venv/lib/python3.12/site-packages
Requires: grpclib, python-dateutil, typing-extensions
Required-by:
```

### Checklist

- [X] I have searched the issues for duplicates.
- [X] I have shown the entire traceback, if possible.
- [X] I have verified this issue occurs on the latest prelease of betterproto which can be installed using `pip install -U --pre betterproto`, if possible.