Updated user cache is ignored

Even if some own changes has been made in user caches, it won't be picked up by fontconfig and still referring system-wide one.

For example, applying the following config to update caches:

<fontconfig>
  <match target="scan">
    <test name="family"><string>Noto Sans Mono</string></test>
    <edit name="spacing" mode="assign"><const>mono</const></edit>
  </match>
</fontconfig>

But fc-match and fc-list doesn't show this. if changing the order of cachedir in fonts.conf as the follow will works:

  <cachedir prefix="xdg">fontconfig</cachedir>
  <cachedir>/var/cache/fontconfig</cachedir>

This explains fontconfig reads caches in the order of cachedir regardless of its timestamps. Note, if we change like this, when running fc-cache by root, all of caches will be stored in /root/.cache/fontconfig. this is completely wrong.