[Feature] Rebase before pushing in auto-pr.ps1 to prevent push conflicts

[abgox]

Feature Request

• Currently, the auto-pr.ps1 script in the Scoop core repository may fail to push changes if new commits are added to the default branch while the GitHub Actions workflow (Excavator) is running.
• This often results in a “non-fast-forward” push error.

Describe the solution you'd like

• Before performing the final push, the script could run a rebase (for example, git pull --rebase) to ensure it’s up to date with the remote repository.

Why it is needed

• Prevents GitHub Actions from failing due to push conflicts.
• Improves reliability of the automated checklist update process.