benchmark: improve cpu.sh for safety and usability #253

Job	Run time
Build slim tarball	1m 36s
aarch64-linux: with shared libraries	25m 9s
aarch64-darwin: with shared libraries	32m 7s
x86_64-darwin: with shared libraries	47m 0s
x86_64-linux: with shared libraries	29m 6s
	2h 14m 58s

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