fix load/save mismatch in missions

When `$Contrail Speed Threshold:` and `+Volumetric Nebula:` are used in a mission, builds since 23.1 would throw an error, since the fields were loaded in a different order than they were saved.  It turned out that the save order was the correct order, so this fixes the load order.

Author: Goober5000

code/mission/missionparse.cpp

@@ -804,15 +804,15 @@ void parse_mission_info(mission *pm, bool basic = false)
 		stuff_float(&Neb2_fog_far_mult);
 	}
 
-	if (optional_string("+Volumetric Nebula:")) {
-		pm->volumetrics.emplace().parse_volumetric_nebula();
-	}
-
 	// Goober5000 - ship contrail speed threshold
 	if (optional_string("$Contrail Speed Threshold:")){
 		stuff_int(&pm->contrail_threshold);
 	}
 
+	if (optional_string("+Volumetric Nebula:")) {
+		pm->volumetrics.emplace().parse_volumetric_nebula();
+	}
+
 	// get the number of players if in a multiplayer mission
 	if ( pm->game_type & MISSION_TYPE_MULTI ) {
 		if ( optional_string("+Num Players:") ) {