Update README to use actions/checkout@v6 instead of v4

[gwynne]

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[cla-bot]

Thank you for your pull request and welcome to the Skip community. We require contributors to sign our contributor license agreement (CLA), and we don't seem to have the user(s) @gwynne on file. In order for us to review and merge your code, for each noted user please add your GitHub username to Skip's .clabot file

[marcprux]

Thanks!