Checkupdates: Preserve pacman colors when printing the list of pending updates

The colors for the listing of pending updates are inherited from the "Color" option in /etc/pacman.conf (since it uses pacman -Qu to generate the list).

However, since the list is put into an array via a mapfile the colors are not preserved. Indeed, the "Color" option in /etc/pacman.conf correspond to the --color auto pacman flag which prints colors only when outputting onto in a TTY (which a mapfile/array is not 😅) and the lack of coloring seems confusing for users (see #117 (closed) & #1 (closed)).

This commit makes checkupdates forcing the use of --color always during the pacman -Qu command in the mapfile to generate the list of pending updates with colors for users that have the "Color" option enabled in the pacman.conf.