chore(CI): update actions/checkout to v4 #1310

Job	Run time
changed_paths	10s
clippy	6m 55s
cargo check (host)	7m 57s
build boot image (x86_64)	7m 42s
Loom tests (cordyceps)	5m 27s
Miri tests (codyceps)	6m 23s
docs	8m 49s
Loom tests (maitake)	8m 9s
Tests (maitake, no-default-features)	5m 49s
rustfmt	21s
cargo test (host)	9m 13s
cargo test (cross x64)	8m 24s
Loom tests (mycelium-util)	4m 41s
	1h 20m 0s

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