proposal: enhance keep_files key

[Rratic]

Checklist

• [X] I am using the latest version of this action.
• [X] I have read the latest README and followed the instructions.
• [X] I have read the latest GitHub Actions official documentation and learned the basic spec and concepts.

Describe your proposal

Not to overwrite the .github dir in target branch.

Describe the solution you'd like

Enhance the keep_files key. Enable values like [".github/*"].

Describe alternatives you've considered

1. Write shell scripts in workflow code.
2. Move workflow code to master branch.

Additional context

I want to build a github pages mirror (while an master => gh-pages builder is set using this), so someone helped me write a workflow in gh-pages branch. But it worked only once, and I found that the workflow was deleted view here, the first box.