NR S02a: restrict movement by costs instead moves

because it is not obvious shown in the sidebar

[ci skip]

data/campaigns/Northern_Rebirth/scenarios/02_01_Infested_Caves.cfg

@@ -760,7 +760,7 @@
             side=2,3,5,6,7,8
             x=24-31
             y=21-27
-            # exclude the ZoC of the unit guarding the entrance 
+            # exclude the ZoC area of the unit guarding the entrance
             [not]
                 x=30,31
                 y=27,27
@@ -799,14 +799,25 @@
     [event]
         name=side 4 turn refresh
         first_time_only=no
-        [modify_unit]
+
+        [object]
+            silent=yes
+            duration=turn end
+            take_only_once=yes
             [filter]
                 side=4
                 x=31
                 y=28
             [/filter]
-            moves=0
-        [/modify_unit]
+            [effect]
+                apply_to=movement_costs
+                replace=yes
+                [movement_costs]
+                    cave={UNREACHABLE}
+                    castle={UNREACHABLE}
+                [/movement_costs]
+            [/effect]
+        [/object]
     [/event]
 
     # Micro AI, controlling a unit which is supporting the one above.