CI-unixish

updated `actions/checkout` to `v6` (#603) #1263

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Run time

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Job	Run time
build (macos-15, clang++)	49s
build (ubuntu-22.04, clang++)	54s
build (macos-14, clang++)	35s
build (macos-15-intel, clang++)	1m 17s
build (ubuntu-24.04, g++)	2m 51s
build (ubuntu-24.04, clang++)	3m 3s
build (macos-26, clang++)	38s
build (ubuntu-22.04, g++)	33s
	10m 40s