refactor(prep-lib): use `sha256sum()` if available #664

Job	Run time
macos-latest (release)	2m 0s
macos-latest with pre-built binary (release)	6s
windows-latest (release)	8m 56s
windows-latest with pre-built binary (oldrel-1)	10s
ubuntu-latest (devel)	1m 56s
ubuntu-latest (release)	2m 20s
windows-latest with pre-built binary (release)	9s
ubuntu-latest (oldrel-1)	1m 51s
windows-latest with pre-built binary (devel)	14s
ubuntu-latest with pre-built binary (oldrel-1)	3s
ubuntu-latest with pre-built binary (release)	7s
ubuntu-latest with pre-built binary (devel)	4s
	17m 56s

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