Currently, when using dune build @fmt to format dune files, we look in the PATH to find a dune executable. There are two issues with this:

1. dune may not be in the PATH
2. if a directory in the PATH happens to contain any file named dune, then this file will be taken to be the dune executable

It seems to me a better way could be to simply use the currently executing dune binary instead of looking for it in the PATH.