test: delay writing the files only on macOS #2678

Job	Run time
Build slim tarball	1m 32s
aarch64-linux: with shared libraries	22m 40s
x86_64-linux: with shared libraries	27m 48s
aarch64-darwin: with shared libraries	30m 58s
x86_64-darwin: with shared libraries	45m 39s
	2h 8m 37s

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