Fix: 1 security vulnerabilities in subdomain.py

[adithyan-ak]

🔒 Security Fixes

This PR fixes 1 security vulnerability:

✅ 1. Hardcoded API Key

Replaced the hardcoded VirusTotal API key with reading the API key from an environment variable 'VT_API_KEY'. Added a check to raise a ValueError if the environment variable is not set.

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