sync.yml: Add stable_6.12.y

This will cause our current github action to also sync the
stable_6.12.y branch with the latest 6.12.y stable tree

LE-2709

Author: bmastbergen

.github/workflows/sync.yml

@@ -17,6 +17,9 @@ jobs:
            - branch: kernel-mainline
              remote: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
              remote_branch: master
+           - branch: stable_6.12.y
+             remote: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
+             remote_branch: linux-6.12.y
 
     env:
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}