DM: S20: Fix waittime at scenario start

Don't animate unit advancements during a prestart event.

It woudl be better it u:advance() would automaticialyl not waste time animating advancements that can't be seen (during prestart events) but for this scenario this is already an improvement

(cherry picked from commit e549b03412)

data/campaigns/Delfadors_Memoirs/utils/sides.cfg

@@ -550,7 +550,7 @@
                 end
                 -- level the unit and give it a random amount of experience up to half of max
                 units[i].experience = units[i].max_experience
-                units[i]:advance( )
+                units[i]:advance(false)
                 units[i].experience = mathx.random ( 0, mathx.round( units[i].max_experience / 2 ) )
             end
         >>