refactor(prep-lib): use `sha256sum()` if available #162

Job	Run time
build (ubuntu-latest, x86_64-unknown-linux-musl)	56s
build (ubuntu-latest, aarch64-unknown-linux-musl)	40s
build (macos-latest, x86_64-apple-darwin)	54s
build (macos-latest, aarch64-apple-darwin)	49s
build (windows-latest, x86_64-pc-windows-gnu)	3m 44s
test (macos-latest, release)	1m 12s
test (windows-latest, oldrel-1)	7m 24s
test (windows-latest, release)	2m 40s
test (windows-latest, devel)	2m 25s
test (ubuntu-latest, oldrel-1)	1m 41s
test (ubuntu-latest, release)	1m 38s
test (ubuntu-latest, devel)	1m 24s
test (macos-12, release)	2m 7s
release	0s
	27m 34s

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