Set modified timestamps based on Git commit

[alerque]

Similar to #36 but for generated artifacts (production font builds, whatever you want to call them) the TTF & OTF files have date stamps in them. This is slightly interesting information but it breaks the concept of reproducible builds. I can't verify that two font builds are exactly the same if they change on every build even when nothing changes.

I propose we set this field on all generated artifacts to the timestamp of the Git commit they were built from.