Mix.Utils.symlink_or_copy always copies on Windows

This is a start to avoiding symlinking on WIndows; fixing this method alone brings the number of test failures down from 13 to 8.  There are other uses of symlinks that need to be handled, namely [Mix.Project.build_structure/2](https://github.com/elixir-lang/elixir/blob/master/lib/mix/lib/mix/project.ex#L306).