The rpath structure for the bootstrap tools was reworked to minimize
the amount of rewriting required on unpack, but the test was not
updated to match the different structure.
Additionally [1] builds that use the bootstrap version of libc++
cannot find libc++abi if the reference includes the "lib"
component (ie, libc++ refers to libc++abi with
@rpath/lib/libc++abi.dylib).
[1] https://logs.nix.samueldr.com/nix-darwin/2021-05-18#4993282
Test failure observed on Hydra: https://hydra.nixos.org/build/143130126
38207735f4