fix: add SSL certificate verification fallback for virtualenv download (closes #28044)

[botbikamordehai2-sketch]

What

The script install.py fails on corporate/proxied networks with self-signed certificates when trying to download virtualenv from PyPI. This causes 'az bicep install' to fail indirectly.

Fix

Add a fallback that retries the download with SSL verification disabled if the first attempt fails due to certificate errors. This matches common patterns used elsewhere in Azure CLI for handling such environments.

Closes #28044

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