Update actions/checkout action to v4 #382

Job	Run time
CheckTargets	1m 24s
Docs	11s
Clippy	13s
Check (aarch64-apple-ios)	29s
MinimalVersions (ubuntu-latest)	19s
Check (aarch64-apple-tvos)	34s
MSRV (ubuntu-latest)	13s
MinimalVersions (macos-latest)	30s
Nightly	24s
Check (aarch64-unknown-openbsd)	33s
MSRV (macos-latest)	22s
MinimalVersions (windows-latest)	1m 2s
Check (arm64_32-apple-watchos)	30s
MSRV (windows-latest)	38s
Check (armv7-sony-vita-newlibeabihf)	23s
Rustfmt	13s
Sanitizer (address)	1m 3s
Check (sparcv9-sun-solaris)	30s
Test (ubuntu-latest)	50s
Sanitizer (leak)	48s
Check (x86_64-pc-solaris)	33s
Test (macos-latest)	2m 15s
TestPoll	34s
Check (x86_64-unknown-dragonfly)	29s
Test (windows-latest)	1m 54s
Check (x86_64-unknown-openbsd)	34s
Sanitizer (memory)	1m 38s
TestWakerPipe	32s
Sanitizer (thread)	54s
Passed	0s
	20m 32s

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