build(deps): bump actions/checkout from 3 to 4 #1199

Job	Run time
Clang Tidy	8s
Clippy	35s
Clippy nightly (optional)	43s
Build+test (clang, stable, ubuntu-20.04)	1m 17s
Build+test (clang, beta, ubuntu-20.04)	1m 25s
Build+test (clang, nightly, ubuntu-20.04)	1m 25s
Build+test (clang, 1.60.0, ubuntu-20.04)	4m 27s
Build+test (gcc, stable, ubuntu-20.04)	1m 0s
Build+test (gcc, beta, ubuntu-20.04)	1m 13s
Build+test (gcc, nightly, ubuntu-20.04)	1m 20s
Build+test (gcc, 1.60.0, ubuntu-20.04)	3m 36s
Build+test (macos-latest, clang, stable)	1m 33s
Check for documentation errors	22s
ensure-header-updated	1m 49s
Format	23s
Check minimum versions	36s
Miri	1m 10s
Windows	2m 42s
Windows CMake, Debug configuration	2m 45s
Windows CMake, Release configuration	2m 22s
Valgrind	1m 34s
	32m 25s

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