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.

Modules/ExternalData.cmake

@@ -945,7 +945,7 @@ function(_ExternalData_link_or_copy src dst)
     file(CREATE_LINK "${tgt}" "${tmp}" RESULT result COPY_ON_ERROR SYMBOLIC)
   else()
     # Create a copy.
-    file(COPY_FILE "${src}" "${tmp}" RESULT result)
+    file(COPY_FILE "${src}" "${tmp}" RESULT result INPUT_MAY_BE_RECENT)
   endif()
   if(result)
     file(REMOVE "${tmp}")