This introduces a ParseSourceDateEpoch function, which can be used
to parse "SOURCE_DATE_EPOCH" values for situations where those
values are not passed through an env-var (or the env-var has been
read through other means).
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
For determinism of human-readable string representation.
e.g., "2023-01-10T12:34:56Z" vs "2023-01-10T21:34:56+09:00"
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>