make-bootstrap-tools: preserve coreutils symlinks

We go out of our way (see top of file) to build a single binary
with symlinks for all of the tools, but were losing them
when preparing the bootstrap tools.

pkgs/stdenv/linux/make-bootstrap-tools.nix

···
     '' else throw "unsupported libc for bootstrap tools")
     + ''
         # Copy coreutils, bash, etc.
-        cp ${coreutilsMinimal.out}/bin/* $out/bin
+        cp -d ${coreutilsMinimal.out}/bin/* $out/bin
         (cd $out/bin && rm vdir dir sha*sum pinky factor pathchk runcon shuf who whoami shred users)
 
         cp ${bash.out}/bin/bash $out/bin