doc(guide): use fresh actions/checkout version in GH actions examples

[AMDmi3]

What does this PR try to resolve?

Update actions/checkout version to the actual one in examples of GitHub actions code

How to test and review this PR?

Check actual action version https://github.com/actions/checkout/tags

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @epage (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ehuss, @epage, @weihanglo
• @ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
• Random selection from ehuss, epage, weihanglo

[weihanglo]

Thanks!

Just FYI. We have #16741 asking for an update. There is some open question about level of support from maintainer for these snippets. My thought on it in #16741 (comment):

IMHO we don't need to be worried about that. Incremental doc improvement is better than nothing. As maintainers we provide the snippet as-is. If contributor comes we accept. If none it falls behind.

If we are worried that we may forget the perfect solution after a simple update, then we keep the issue open.

View changes since this review