Improve update-schedule workflow to prevent branch divergence

[perashanid]

This PR improves the update-schedule.yml workflow to prevent future failures caused by diverged branches.

PROBLEM:
The workflow has been failing because:

1. The update-schedule branch has diverged from main
2. Branch protection rules prevent force-pushing
3. The workflow cannot update the branch, causing silent failures

CHANGES MADE:

• Added explicit ref: main to the checkout step to ensure the workflow always starts from the latest main branch
• This ensures the workflow creates changes based on the current main branch state

IMPORTANT NOTE:
This PR alone will not fix the current failure. A maintainer with appropriate permissions needs to delete the stale update-schedule branch first. Once deleted, this improved workflow will:

1. Start fresh from main
2. Create a new update-schedule branch with the latest changes
3. Prevent future divergence issues

TESTING:
After the stale branch is deleted by a maintainer:

• The workflow should run successfully
• It will create a new PR with schedule updates
• Future runs will work correctly

Fixes #54940

[k8s-ci-robot]

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• .github/workflows/OWNERS

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