CI image builds failed repeatedly because GitHub releases dropped
connections (RemoteDisconnected) on all 5 retry attempts. Add
ghproxy.com as a fallback URL for _download_and_extract_tarball so
the build can fall through to a mirror when GitHub is flaky.
- Add fallback_urls parameter to _download_and_extract_tarball
- Add ghproxy.com mirror fallback for actionlint and vale
- Add tests for fallback success and all-URLs-fail paths
DEVX-159
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
