For those, like me, who can't find the final rooftop race for the life of them, try looking at the top of the Science Centre in Central Shade.
It is right next to the Science Centre icon. You may have to zoom in and out a few times to actually get it to appear on the map.
Side note: It is an agility level 4 race.
Edit: This video shows the locations of every rooftop race, and how to get gold in them.
The races that need to be completed are as follows:
Thanks to z0rk for finding this video and, obviously, cakechieveables on YouTube for making it.