GraphCopy: fix Windows export_clean_symbol for __Nil and __None (again)
In !19 (merged) I did not take the two extra
__ characters for
__None into account. The sizes of the strings should be 5 and 6, not 3 and 4.
This probably causes https://gitlab.com/clean-and-itasks/abc-interpreter/-/issues/113, but I have no way to test at the moment.