Respect SOURCE_DATE_EPOCH env var for modified time
Respect SOURCE_DATE_EPOCH env var for modified time

For reproducible builds, see:
https://reproducible-builds.org/specs/source-date-epoch/

Fixes https://github.com/googlei18n/ufo2ft/issues/161

googlefonts/ufo2ft,jamesgk/ufo2fdk,googlei18n/ufo2ft,jamesgk/ufo2ft,moyogo/ufo2ft