Motivation for this change

When loading pbdZMQ on Darwin, R tries to find otool and install_name_tool dynamically; R invokes system("which otool", ...) and system("which install_name_tool", ...).
In a pure environment (nix-shell --pure), even if having these dependencies in buildInputs of pbdZMQ, they are not visible from the caller, somehow, so that it fails.
Even worse in an impure environment, R calls /usr/bin/otool. Without installation of Xcode or Command Line Tools, you'll see a popup message saying "would you like to install the tools now?"

Things done

• Replaced each system("which ...") call by a static path to each dependency.
• Tested it using the following script:

#!/usr/bin/env nix-shell --pure

with import <nixpkgs> {};
stdenv.mkDerivation {
  name = "darwin-r-pbdZMQ-test";
  buildInputs = [ (rWrapper.override { packages = [ rPackages.pbdZMQ ]; }) ];
  shellHook = ''
    Rscript -e 'library(pbdZMQ)'
    exit 0
  '';
}

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.