jsoncpp: don't rely on coreutils cp

pkgs/development/libraries/jsoncpp/default.nix

···
     sha256 = "0p92i0hx2k3g8mwrcy339b56bfq8qgpb65id8xllkgd2ns4wi9zi";
   };
 
+  /* During darwin bootstrap, we have a cp that doesn't understand the
+   * --reflink=auto flag, which is used in the default unpackPhase for dirs
+   */
+  unpackPhase = ''
+    cp -a ${src} ${src.name}
+    chmod -R +w ${src.name}
+    export sourceRoot=${src.name}
+  '';
+
   nativeBuildInputs = [
     # cmake can be built with the system jsoncpp, or its own bundled version.
     # Obviously we cannot build it against the system jsoncpp that doesn't yet exist, so