ci: Update to `actions/checkout@v4` from `v2`. #117

Job	Run time
build (ubuntu-latest, Debug, address)	24s
build (ubuntu-latest, Debug, memory)	25s
build (ubuntu-latest, Debug, undefined)	34s
build (ubuntu-latest, RelWithDebInfo, address)	32s
build (ubuntu-latest, RelWithDebInfo, memory)	38s
build (ubuntu-latest, RelWithDebInfo, undefined)	36s
build (ubuntu-latest, MinSizeRel, address)	30s
build (ubuntu-latest, MinSizeRel, memory)	26s
build (ubuntu-latest, MinSizeRel, undefined)	34s
build (ubuntu-latest, Release, address)	38s
build (ubuntu-latest, Release, memory)	1m 12s
build (ubuntu-latest, Release, undefined)	43s
build (macOS-latest, Debug, address)	26s
build (macOS-latest, Debug, undefined)	24s
build (macOS-latest, RelWithDebInfo, address)	50s
build (macOS-latest, RelWithDebInfo, undefined)	55s
build (macOS-latest, MinSizeRel, address)	45s
build (macOS-latest, MinSizeRel, undefined)	40s
build (macOS-latest, Release, address)	32s
build (macOS-latest, Release, undefined)	38s
	12m 22s

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