CI: Use gh-cli for changed files, and workaround codespell skip list bug

For PRs, this should give a more accurate list, as the previous method would
diff to the tip of the `master` branch, which could include new commits (and
thus changed files) not present in the PR branch.

codespell's `--skip` option doesn't work at all with folders when used
together with an explicit list of paths to work with, so let's not use it.