Use gh auth setup-git to initialize git authentication

Closes #248

GitOrigin-RevId: 212427a
Change-Id: I4e5d3a82e96cb5d88c18c1cbf42fbd6c9bb44d9c
Reviewed-on: https://dart-review.googlesource.com/c/dart-docker/+/452760
Commit-Queue: Alexander Thomas <[email protected]>
Reviewed-by: Ivan Inozemtsev <[email protected]>

Author: athomas

.github/workflows/publish-library.yml

@@ -30,6 +30,7 @@ jobs:
           set -x
           git config --global user.name "dart-github-bot"
           git config --global user.email "[email protected]"
+          gh auth setup-git
 
           gh repo sync dart-github-bot/official-images
           gh repo clone dart-github-bot/official-images