70-fonts-noto-cjk.conf: Increase font priority on Chinese systems

As previously discussed on the Chinese list, we want to make Noto CJK fonts
the "default" one on all Chinese environments, or at least on those Chinese
locales that are supported by Noto CJK fonts.

This commit would use prepend_first instead of prepend for Noto CJK
fonts on Chinese locale systems, which ensures the selection of Noto CJK
fonts.

Signed-off-by: Boyuan Yang <073plan@gmail.com>