Implement support for the SOURCE_DATE_EPOCH environment variable as
specified by reproducible-builds.org. When set, this variable overrides
the current timestamp in the Release file's Date and Valid-Until fields,
enabling reproducible filesystem publishes.
- Read SOURCE_DATE_EPOCH environment variable in Publish()
- Use the epoch timestamp for both Date and Valid-Until fields
- Gracefully fallback to current time if unset or invalid
- Add comprehensive tests for valid and invalid SOURCE_DATE_EPOCH values
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