ci: Update to `actions/checkout@v4` from `v2`. #118

Job	Run time
build (ubuntu-latest, Debug, address)	27s
build (ubuntu-latest, Debug, memory)	25s
build (ubuntu-latest, Debug, undefined)	31s
build (ubuntu-latest, RelWithDebInfo, address)	34s
build (ubuntu-latest, RelWithDebInfo, memory)	31s
build (ubuntu-latest, RelWithDebInfo, undefined)	35s
build (ubuntu-latest, MinSizeRel, address)	28s
build (ubuntu-latest, MinSizeRel, memory)	34s
build (ubuntu-latest, MinSizeRel, undefined)	35s
build (ubuntu-latest, Release, address)	25s
build (ubuntu-latest, Release, memory)	30s
build (ubuntu-latest, Release, undefined)	31s
build (macOS-latest, Debug, address)	37s
build (macOS-latest, Debug, undefined)	23s
build (macOS-latest, RelWithDebInfo, address)	33s
build (macOS-latest, RelWithDebInfo, undefined)	49s
build (macOS-latest, MinSizeRel, address)	36s
build (macOS-latest, MinSizeRel, undefined)	52s
build (macOS-latest, Release, address)	43s
build (macOS-latest, Release, undefined)	46s
	11m 25s

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