Default Git commit author and committer to the user triggering the CI

For consistency, we want Git to use the name of the triggering user everywhere. Unless we do that, it will use .git/config in some places, EMAIL and "root" in other places, while debian/changelog consistently uses the user’s name and email anyway.

If we decide we want the automatically scheduled jobs to have something vague like "Apertis CI", we can override EMAIL and NAME in the schedule definition.