Fix #10334: Store separate newgrf-safe version of date_of_last_service. (#11124)

This value is not changed when the date cheat is used, which caused issues with changing properties based on service date.

Co-authored-by: Peter Nelson <peter1138@openttd.org>
(cherry picked from commit 9a602ff304)

src/roadveh_cmd.cpp

@@ -311,6 +311,7 @@ CommandCost CmdBuildRoadVehicle(TileIndex tile, DoCommandFlag flags, const Engin
 		v->SetServiceInterval(Company::Get(v->owner)->settings.vehicle.servint_roadveh);
 
 		v->date_of_last_service = _date;
+		v->date_of_last_service_newgrf = _date;
 		v->build_year = _cur_year;
 
 		v->sprite_seq.Set(SPR_IMG_QUERY);