ExternalData: Improve robustness on Windows to copy a data object to a file

When an external data object was recently created on disk, perhaps
fetched by the current process or another process, it may be
inaccessible on Windows for a short time.  Tell our `file(COPY_FILE)`
call to retry a few times to copy the object to the final file path.

Hopefully this will resolve our long-standing spurious failures of the
`Module.ExternalData` test on Windows.