chore: update actions/checkout action to v4 #102

Job	Run time
cargo check	39s
cargo check (+MSRV -Zminimal-versions) (1.65.0)	1m 53s
cargo check (feature combinations)	18s
cargo check (+MSRV -Zminimal-versions) (stable)	27s
cargo fmt	12s
cargo test (stable on ubuntu-latest)	1m 4s
build tests (wasm)	59s
warnings	41s
cargo test (beta on ubuntu-latest)	1m 4s
cargo test (nightly on ubuntu-latest)	1m 4s
cargo test (stable on windows-latest)	2m 43s
cargo test (stable on macos-latest)	2m 15s
all systems go!	1s
	13m 20s

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