ci: bump actions/checkout from 3.5.3 to 4.0.0 #461

Job	Run time
no_std build (stable)	2m 4s
no_std build (nightly)	2m 38s
Build and check benchmarks	2m 23s
Linux/ARM - Release tests (i686-unknown-linux-gnu)	23s
Linux/ARM - Release tests (armv7-unknown-linux-gnueabihf)	24s
Linux/ARM - Release tests (mips64-unknown-linux-gnuabi64)	19s
Build documentation	45s
Tests w. sanitizers	2m 55s
test (stable, ubuntu-latest)	1m 47s
test (stable, macos-latest)	1m 59s
test (stable, windows-latest)	1m 39s
test (beta, ubuntu-latest)	2m 32s
test (beta, macos-latest)	2m 33s
test (beta, windows-latest)	2m 33s
test (nightly, ubuntu-latest)	2m 34s
test (nightly, macos-latest)	2m 2s
test (nightly, windows-latest)	2m 3s
test (1.64.0, ubuntu-latest)	2m 34s
test (1.64.0, macos-latest)	2m 33s
test (1.64.0, windows-latest)	2m 29s
WebAssembly - Release build (wasm32-unknown-unknown)	28s
	39m 37s

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