openscad: do not remove CLI for Darwin

In its prior form, OpenSCAD would be made available via a typical macOS `.app`
bundle.  OpenSCAD also provides a useful command line interface that is helpful
for automation and other scripted activities.  The `openscad` binary used by the
`.app` bundle works as the CLI.  These changes keep the original `openscad`
binary and add it to `bin` where it will automatically wind up on the `PATH` for
consumers of this package.

pkgs/by-name/op/openscad/package.nix

@@ -168,7 +168,7 @@ stdenv.mkDerivation rec {
   postInstall = lib.optionalString stdenv.hostPlatform.isDarwin ''
     mkdir $out/Applications
     mv $out/bin/*.app $out/Applications
-    rmdir $out/bin || true
+    ln -s "$out/Applications/OpenSCAD.app/Contents/MacOS/OpenSCAD" "$out/bin/openscad"
 
     mv --target-directory=$out/Applications/OpenSCAD.app/Contents/Resources \
       $out/share/openscad/{examples,color-schemes,locale,libraries,fonts,templates}