Fix pi_netrw.txt build rule to be portable to macOS #932

Job	Run time
build-and-test (macos-13, 15.2, true, vimtags, check-xcodeproj-compat) / build-and-test	11m 54s
build-and-test (macos-15, 16.2, true, true, true) / build-and-test	23m 43s
build-and-test (macos-14, 15.4, false) / build-and-test	10m 6s
build-and-test (macos-13-xcode14-self-hosted, true, _10.9, true, true) / build-and-test	0s
	45m 43s

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