Keep configurator logs on Windows

On Windows (mingw-w64 in a Cygwin environment), dune-configurator
writes object files and logs output of the compiler in a directory in
/tmp/buildXXXXXX.dune/ocaml-configuratorYYYYYY/. That directory is
deleted when the build finishes and the logs are lost. It's a problem
when implementing a configurator/discover script, because it's harder
to debug it without the compiler's output.

I suggest that either the configurator output could be retained and
its location shown to the user, or that it could be duplicated in the
_build directory, or that it could be shown with a flag.

I think it could be as simple as removing otherlibs/configurator/src/v1.ml#L50
and the rm_rf function from otherlibs/configurator/v1.ml.
(EDIT: apparently not) This is a temp directory after all. Even if it isn't
cleaned by Cygwin, there's no extreme pressure for Dune to remove it itself.

Start with otherlibs/configurator/src/v1.ml around the linked cleanup line and otherlibs/configurator/v1.ml around rm_rf. Trace how the temporary configurator directory and compiler logs are created and removed on Windows/Cygwin. Done means choosing and documenting a supported way to retain or expose those logs for configurator and discover scripts.