[security] Move s3-html-update workflow into its own environment (#107889)
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### <samp>🤖 Generated by Copilot at d02bfb0</samp>
Add environment name for S3 HTMLs workflow. This allows secure and controlled access to the secrets and approval for updating the PyTorch whl indexes on S3.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107889
Approved by: https://github.com/huydhn
