ci: sync GitHub build images with CircleCI

[yanghang8612]

Summary

• align the GitHub Actions Linux build image with CircleCI's ubuntu2404-6 image
• align the GitHub Actions Emscripten build image with CircleCI's emscripten-21 image
• keep both images pinned to the same digests used by .circleci/config.yml

Root cause

PR #133 raised the minimum Boost version to 1.83 and updated the CircleCI and Emscripten script images, but .github/workflows/build.yml still used the older images. The Emscripten job therefore ran with Boost 1.75 and failed during CMake configuration.

Failed build: https://github.com/tronprotocol/solidity/actions/runs/29410477729/job/87336095268

Impact

The Emscripten build can use the required Boost 1.83 toolchain, and the GitHub Actions Linux/Emscripten environments no longer drift from CircleCI.

Validation

• parsed .github/workflows/build.yml successfully as YAML
• ran git diff --check
• verified both pinned GHCR image manifests resolve successfully
• confirmed the image names and digests exactly match .circleci/config.yml

actionlint was not available locally.

[github-actions]

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