Update PyTorch docker base image to Ubuntu-20.04 (take 2) (#101310)
Followup after https://github.com/pytorch/pytorch/pull/101301
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### <samp>🤖 Generated by Copilot at 219c58a</samp>
> _`BASE_RUNTIME` changed_
> _Ubuntu twenty oh four_
> _Spring of new features_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101310
Approved by: https://github.com/atalman
