Remove every file when cleaning up PR preview

[EndBug]

Closes #15

Since JamesIves/github-pages-deploy-action does not currently provide a way to disable checks for .nojekyll and CNAME (see https://github.com/JamesIves/github-pages-deploy-action/discussions/1210), I've opted for running my add-and-commit action on temp worktree. In the future, I may add an input that allows add-and-commit to create the worktree by itself, but for now, I'm doing it in two separate steps.

I've already tested the changes

Let me know what you think 😄