pkg/chrootarchive: fix Darwin build
Before this commit, `doPack`, `doUnpack` and `doUnpackLayer` were not implemented for Darwin, causing build failure.
This change allows all non-Linux Unixes to use FreeBSD reexec-based pack/unpack implementation
See also: moby/buildkit#4059
See also: 8b843732b36541b8b8a11ecb83cf5d51837ae22f
Signed-off-by: Marat Radchenko <marat@slonopotamus.org>