To unlock Street Races you need to complete a number of head to head races - to do so simply challenge Drivatars on the road until they unlock. Once unlocked go to the unlocked event and complete the event to win the Porsche Carrera GT. You do NOT need to win this event for the car to unlock :)
Just to clarify, you have to complete a Street Race NOT a head to head race.
Also it's been brought to my attention that DLC cars completing races may be glitching out so it may be easier to unlock at the moment using a Non-DLC car. - Thanks MushaConvoy
Here's a short video guide you can find this challenge at 1:45