[roll] Configure user.name and user.email

Follow-up to https://dart-review.googlesource.com/c/recipes/+/157960.

user.name and user.email need to be set to allow the service account to
push a merge commit.

TBR=whesse@google.com

https://github.com/dart-lang/sdk/issues/42917

Change-Id: I0c8cfbfc0e567e66c2d1f99f446a434aa9719d6a
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/158040
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
3 files changed