ci: force UTC release build numbers

[ygsgdbd]

Summary

• generate release build numbers with date -u
• keep timestamp build numbers stable even if the runner system timezone changes

Validation

• verified the UTC expression was absent before the change and present afterward
• confirmed TZ=Pacific/Honolulu and TZ=Asia/Shanghai produce the same date -u value
• parsed .github/workflows/release.yml successfully with Ruby YAML
• git diff --check

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