plugins/ciel-switchmirror: minor lint ...

... on the if ... $? syntax

Author: liushuyu

plugins/ciel-switchmirror

@@ -5,8 +5,7 @@ export CIEL_INST="ciel--switchmirror--"
 
 ciel add "$CIEL_INST"
 if [ "$1" != "" ]; then
-ciel shell oma mirror add "$1"
-    if [ "$?" != "0" ]; then
+    if ! ciel shell oma mirror add "$1"; then
         echo -e "\e[1m\e[91m[switch-mirror]:\e[0m\e[1m Failed to set mirror '$1'.\e[0m"
         echo -e "\e[1m\e[91m[switch-mirror]:\e[0m\e[1m More information please refer to the error oma had reported.\e[0m"
         exit 1