Honor the SOURCE_DATE_EPOCH variable

[hongxu-jia]

Implement the SOURCE_DATE_EPOCH specification[1] for reproducible builds.
If SOURCE_DATE_EPOCH is set, use it as timestamp instead of the current time.

[1] https://reproducible-builds.org/specs/source-date-epoch/

[github-actions]

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[mrmhodak]

@pgmpablo157321 to take a look