Update actions/checkout action to v4

[taiki-e]

No description provided.

[taiki-e]

The CI failure seems to be a regression of the vita support introduced in the recent nightly (by rust-lang/rust#119026).

https://github.com/tokio-rs/mio/actions/runs/7432139850/job/20223601302?pr=1752#step:5:51

error[E0425]: cannot find value `SOMAXCONN` in crate `libc`
    --> /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/os/unix/net/listener.rs:87:48
     |
  87 |             const backlog: libc::c_int = libc::SOMAXCONN;
     |                                                ^^^^^^^^^ not found in `libc`

[Thomasdezeeuw]

Thanks @taiki-e