Improve indirect variable expansion in quickstart.sh

[singhhnitin]

Description

This PR improves indirect environment variable expansion in quickstart.sh
by introducing a temporary variable for safer API key detection.
It preserves the existing behavior while making detection more reliable.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #5483

Changes Made

• Introduced a temporary variable env_var for indirect expansion
• Replaced direct array expansion inside the condition check
• Preserved existing output behavior

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (cd core && pytest tests/)
• Lint passes (cd core && ruff check .)
• Manual testing performed

The quickstart script was executed locally to confirm that:

• Existing API keys are detected correctly
• Menu behavior remains unchanged

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes