darwin/make-bootstrap-tools: move "lib" from install name to rpath

[thefloweringash]

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

Motivation for this change

Fix channel blocking bootstrap tools test.

Tested the test attribute of bootstrap tools, but not the resulting full stdenv rebuild.

cc @toonn

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Added a release notes entry if the change is major or breaking
• Fits CONTRIBUTING.md.

[toonn]

This seems to have solved two of my problems. I cherry-picked this commit then generated an LLVM 11 bootstrap-tools (from LLVM 7 bootstrap-tools and stdenv, this part already worked). I then used this LLVM 11 bootstrap-tools in generating another LLVM 11 bootstrap-tools, this time based on an LLVM 11 stdenv. Another step closer to LLVM 11 stdenv for x86_64 darwin! \o/