Improve interaction of the preferLocal, node and nodePath options #815

Right now:

1. If the node is true, the nodePath option is used to create the process.
2. If the preferLocal is true, the nodePath option is used in the child process itself.
3. If the preferLocal is true, local binaries can be run.

Using preferLocal for 2 is a little unexpected. It seems like we should be using node: true instead since this relates to running Node.js and the nodePath option. Also, running local binaries is rather unrelated to choosing the Node.js version. It is odd to couple them.

On the other hand, by making 2 use node: true instead, we would be ensuring that the nodePath option is always used both to create the process and in the child process itself. For example, execaNode() currently uses the nodePath option in one case but not the other. Users should expect a consistent Node.js version both in the child process and the "grand-child" processes.

What do you think?