Nix -- how to use addAutoPatchelfSearchPath correctly?

I'm packaging a tool that includes some shared objects natively, and I think the right approach to add those objects to the rpath is to use the addAutoPatchelfSearchPath function to the installPhase of my Nix expression, but it's not picking up the .so libs as expected. I've tried setting the search path explicitly, relatively, etc. What would the correct usage be?

installPhase = ''
    mkdir -p $out
    addAutoPatchelfSearchPath $(pwd)
    cp -r $src $out
''