Bump actions/checkout from 3 to 4 #4004

Job	Run time
Bitrot check	2m 46s
Book tests	3m 14s
Code coverage	41m 44s
Build target wasm32-unknown-unknown	2m 53s
Build target wasm32-wasi	2m 56s
Intra-doc links	3m 19s
Rustfmt	21s
Test on ubuntu-latest	2m 39s
Test on windows-latest	2m 32s
Test on macOS-latest	2m 35s
Test on ubuntu-latest with beta features	2m 35s
Test on windows-latest with beta features	2m 38s
Test on macOS-latest with beta features	2m 31s
Test on ubuntu-latest with nightly features	2m 42s
Test on windows-latest with nightly features	2m 36s
Test on macOS-latest with nightly features	2m 36s
Test on i686-unknown-linux-gnu	3m 14s
Test on i686-unknown-linux-gnu with beta features	3m 12s
Test on i686-unknown-linux-gnu with nightly features	3m 1s
	1h 30m 4s

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