Make Model Name Configurable via Environment Variables

[kanak8278]

Currently, the model name is hard-coded in the mini_llm.py file. To enhance flexibility and support different model configurations without modifying the code, it would be beneficial to make the model name configurable via environment variables.

https://github.com/microsoft/PwR-NL2DSL/blob/afc335223e6c90059363c7bc749c9d0eada45213/nl2dsl/utils/mini_llm.py#L18C1-L19C1