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)
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### 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
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### 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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Beginne in tutorial/build_index.py beim fest codierten Indexnamen und vergleiche ihn mit dem Eintrag AZUREAI_SEARCH_INDEX_NAME in tutorial/sample.env. Verfolge, wie die Umgebungskonfiguration geladen wird, und überprüfe anschließend, dass der konfigurierte Name verwendet wird und der vorhandene Standardwert weiterhin wirksam ist, wenn kein Wert angegeben wird.

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