Static AutoTarget nextScanTime to combat desyncs

This fix removes the random number generator, too and assumes actors won't scan for new targets all at once in non-scripted missions anyway because they start ticking upon placement into the world which is already fairly random. It seems that the auto-target desync problem persists in the last playtest releases https://github.com/OpenRA/OpenRA/issues/2763#issuecomment-14826367.