tools: update checkout action on CI

[UltiRequiem]

https://github.com/actions/checkout#whats-new
Updated to the node16 runtime by default.

[rust-highfive]

r? @Mark-Simulacrum

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.debug-assertions := True
configure: rust.overflow-checks := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
   Compiling thiserror v1.0.30
   Compiling yaml-merge-keys v0.4.1
   Compiling expand-yaml-anchors v0.1.0 (/checkout/src/tools/expand-yaml-anchors)
    Finished release [optimized] target(s) in 4.90s
error: .github/workflows/ci.yml is not up to date; please run `x.py run src/tools/expand-yaml-anchors`.
caused by: src/ci/github-actions/ci.yml and .github/workflows/ci.yml are different

[UltiRequiem]

I just learn about https://github.com/rust-lang/rust/blob/master/src/ci/github-actions/ci.yml

I will make a new PR with the required changes.