ci: consistently use checkout@v4 action #184

Job	Run time
Clippy (Nightly)	11s
Clippy	11s
Build+test (stable, ubuntu-20.04)	1m 1s
Format	4s
Check minimum versions of direct dependencies	16s
Build+test (beta, ubuntu-20.04)	1m 5s
Build+test (nightly, ubuntu-20.04)	1m 3s
msrv (ubuntu-20.04)	3m 56s
Check semver compatibility	17s
Build+test (macos-latest, stable)	2m 22s
msrv (macos-latest)	8m 59s
Build+test (windows-latest, stable)	3m 0s
msrv (windows-latest)	5m 18s
	27m 43s

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