scripts: coccicheck: Add quotes to improve portability

While fetching the number of threads per core with lscpu,
the [:digit:] set is used for translation of digits from 0-9.
However, using [:digit:] instead of "[:digit:]" does not seem
to work uniformly for some shell types and configurations
(such as zsh).

Therefore, modify coccicheck to use double quotes around the
[:digit:] set for uniformity and better portability.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>