Make the build reproducible

While auditing Debian's packages, Chris Lamb reported[*] that
weather-util's correlation set generation is not reproducible
because it embeds timestamps without a means to override them and
also varies by system timezone. Allow SOURCE_DATE_EPOCH from the
calling environment and assume UTC rather than relying on locale
settings when no timezones are specified.

[*] https://bugs.debian.org/964721