drone: Fix one remaning "$F".

.drone.yml

@@ -23,7 +23,7 @@ steps:
       - ln -s $DIR/_site $DIR.link
       - mv -T $DIR.link static
       - for G in dir.*; do
-      - if [ ! "$G" -ef "$F" ]; then rm -rf "$G"; fi
+      - if [ ! "$G" -ef "$DIR" ]; then rm -rf "$G"; fi
       - done
     volumes:
       - name: hosting