fix: Use pgrep instead of grepping ps output

addresses shellcheck SC2009

src/script/arch-update.sh

@@ -759,7 +759,7 @@ case "${option}" in
 				state_up_to_date
 			fi
 
-			if ps -ef | grep "[a]rch-update-tray"; then
+			if pgrep -f arch-update-tray > /dev/null; then
 				error_msg "$(eval_gettext "There's already a running instance of the Arch-Update systray applet")"
 				exit 3
 			fi