Fix the sync-template action to work

[JessicaS11]

After all the great additions last week, I tested the sync-to-template action on the tester repo. I ended up finding a few issues, fixed herein, as well as discovered a minor bug in the action itself. The PR opened by the action: https://github.com/ICESAT-2HackWeek/jupyterbook-template-tester/pull/2

A couple decision points:

• Currently, we have cookiecutter.yaml as one of our excluded files because it gets filled in with event info. But, it got updated in the last week's edits. I'm inclined to think the case where an excluded file is updated is not worth a custom solution (it would be easy enough to simply edit or delete the .templatesyncignore file in the templated repo and run the action to open a PR with any changes). Thoughts? UPDATE: make any such changes manually or by triggering the update action from a branch with a modified .templatesyncignore file.
• There is a flag in the action to check for existing, open PRs created by the action and "clean them up" (it's recommended to use with caution). Should we leave the default for now (which won't clean them up; I'm not sure if a new PR will be opened or it will just exit, with or without error)? UPDATE: we tried the is_pr_cleanup flag and it did not modify the open PR, just exited the action since the branch existed, so we're leaving as default.