Should traverse up directories looking for `dotnetexec` file

[justin-caldicott]

I was hoping we'd be able to exec from any folder in the repo, but I only appear to be able to exec from the root of the repo, in the folder where the dotnetexec file lives.

It would be great if it traversed up the directory tree looking for the file, like e.g. the dotnet-tools.json itself.

[ivanparvaresh]

@justin-caldicott Thanks, Added to the board