docs: scripts/check-variable-fonts.sh: Improve commands for detection

As mentioned in "Assumption:", current grep expression can't catch
font files whose names are changed from upstream "Noto CJK fonts".

To avoid false negatives, use command of the form:

    fc-list : file family variable

, where ":" works as a wildcard pattern.

Variable fonts can be detected by filtering the output with
"variable=True" and "Noto CJK" font-family variants.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/c62ba2e6-c124-4e91-8011-cb1da408a3c5@gmail.com