fix(install) Make wal2json copy more robust (#1024)

There is a potential conrner case where we may end up with the wal2json library in the postgres/wal2json/VERSION/file but not in postgres/wal2json/wal2json.so.

Not sure exactly how likely this could be, but thechnically it is possible that the download succeeds and cp "../postgres/wal2json/$VERSION/$FILE_NAME" "$FILE_TO_USE" does not. The next attempt the copy would not be attempted.

This fix ensures the copy always happens