use xcode `ar` and `ranlib` if present #254

Job	Run time
local-edit-tests	29s
unavailable	15s
unit-tests	13s
stark.com/foo (linux)	37s
toolchain.com (linux)	1m 22s
pyapp.com/pip (linux)	47s
pyapp.com/poetry (linux)	1m 19s
stark.com/foo@1.2.3 (linux)	32s
git-clone.com (linux)	39s
pc-cmake.com (linux)	36s
fix-machos.com (linux)	34s
version-transformer.com (linux)	35s
stark.com/foo (mac)	55s
toolchain.com (mac)	52s
pyapp.com/pip (mac)	1m 16s
pyapp.com/poetry (mac)	1m 9s
stark.com/foo@1.2.3 (mac)	52s
git-clone.com (mac)	45s
pc-cmake.com (mac)	49s
fix-machos.com (mac)	58s
version-transformer.com (mac)	55s
	16m 29s

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