Cannot build on macOS

I followed your developer guide and attempted to build the project on macOS by using:

```sh
./build.sh -- /p:SkipTests=true
```

which resulted in the following error:

```sh
/wcf/external/runtime/runtime.depproj(78,5): error MSB3073: The command "chmod +x /wcf/bin/testhost/netcoreapp-OSX-Debug-x64/dotnet" exited with code 1.
```

(I've replaced the real absolute path with ``).

This fails, because there is no such file in the targeted directory.

I've tried to run the build on Windows and it runs without any problems.