ci: bump actions/checkout to v4

[trivikr]

Required for using Node.js 20.x in CI

• Changelog for actions/checkout@v4 https://github.com/actions/checkout/blob/main/CHANGELOG.md?rgh-link-date=2024-09-04T18%3A38%3A10Z#v400
• GitHub Blog post https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

[rustbot]

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

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[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Portable SIMD is developed in its own repository. If possible, consider making this change to rust-lang/portable-simd instead.

cc @calebzulawski, @programmerjake

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

The Miri subtree was changed

cc @rust-lang/miri

[saethlin]

This repo already uses actions/checkout@v4, all you did was change the CI configs for a bunch of subtrees, which is a bad idea because those subtrees would get a surprise CI config change when they sync changes back from this repo. Syncing changes from this repo back into a subtree is expected to not fail, because it doesn't contain this kind of shenaniganry.

[trivikr]

@saethlin Thanks for detailed response.

I posted PR after searching for usage of Node.js 16.x in popular GitHub repositories, and didn't notice that this change was in subtrees.

I've posted change in CI config rustc_codegen_gcc rust-lang/rustc_codegen_gcc#560 to check if it gets accepted.