chore: update Docker base image registry from meta-pytorch to huggingface

[sergiopaniego]

Summary

Updates all Docker base image references from ghcr.io/meta-pytorch/openenv-base:latest to ghcr.io/huggingface/openenv-base:latest across all env Dockerfiles, READMEs, docs, CLI template, examples, and scripts.
Also fixes a chat_env build failure (mkdir -p /.cache before pre-downloading GPT-2 tokenizer) and adds fail-fast: false to the build matrix so a single env failure doesn't cancel all other builds.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation
• New environment
• Refactoring

Alignment Checklist

Before submitting, verify:

• I have read .claude/docs/PRINCIPLES.md and this PR aligns with our principles
• I have checked .claude/docs/INVARIANTS.md and no invariants are violated
• I have run /pre-submit-pr (or bash .claude/hooks/lint.sh and tests) and addressed all issues

RFC Status

• Not required (bug fix, docs, minor refactoring)

Test Plan

Images successfully built and pushed to ghcr.io/huggingface/ on the PR branch: build run

Packages now live at: https://github.com/orgs/huggingface/packages?repo_name=OpenEnv

The openspiel-specific base image (openenv-openspiel-base:sha-e622c7e) is intentionally left pointing to meta-pytorch pending a separate rebuild.

Claude Code Review

N/A

[bot-ci-comment]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.