build_index.py: index_name hardcoded in python file, .env AZUREAI_SEARCH_INDEX_NAME is ignored

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### This issue is for a: (mark with an `x`)
```
- [x] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
> in build_index.py the index name is hardcoded to "tutorial-index". However the index can also be specified via the .env file.
IMO the .env file should be read and only if there is no value, we should set a default value (to e.g. "tutorial-index)

hard coded value: https://github.com/Azure-Samples/rag-data-openai-python-promptflow/blob/7e8e6b04727364b2a608fd62b2efa1ffa322d266/tutorial/build_index.py#L44
env specified value: https://github.com/Azure-Samples/rag-data-openai-python-promptflow/blob/7e8e6b04727364b2a608fd62b2efa1ffa322d266/tutorial/sample.env#L12

### Any log messages given by the failure
>

### Expected/desired behavior
> index name should be read from value specified in the .env file

### OS and Version?
> Windows 7, 8 or 10. Linux (which distribution). macOS (Yosemite? El Capitan? Sierra?)

### Versions
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### Mention any other details that might be useful

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> Thanks! We'll be in touch soon.

Start in tutorial/build_index.py at the hardcoded index name and compare it with the AZUREAI_SEARCH_INDEX_NAME entry in tutorial/sample.env. Trace how the environment configuration is loaded, then verify that the configured name is used and that the existing default remains effective when no value is supplied.

Written by the indexing model from the issue text.