ci: Update to `actions/checkout@v4`.

[waywardmonkeys]

This fixes annotations on task runs like the following:

The following actions uses node12 which is deprecated and will be forced
to run on node16: actions/checkout@v2. For more info:
https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

[hsutter]

Thanks!