Vehicle control systems are overwhelmingly programmed in C, mostly from graphical design tools such as MATLAB Simulink via an automatic process. These are real time control systems which are quite different to an interrupt based operating system such as Linux. The many individual controllers must work in concert according to a strict architecture definition and timing schedule that defines the functionality of the vehicle. It's not at all like a PC or phone, whose OS become irrelevant over time, with respect to their environment of other systems. The vehicle environment is the same environment that we inhabit i.e. the one with gravity, friction, charge and the other SI units. This is slowly changing with advent of self driving but, yeah.

God no