Deriving variance of b0

I quote Stock and Watson ("Introduction to econometrics" 4th edition - pag. 101, Key Concept 4.4):

Introduction to Econometrics

The problem is that there is nowhere a proof of this formula: authors don't prove it, and it seems to be impossible to find it online (forums, university textbooks like Gujarati etc.). So, anyone who might help me to derivate it is very gratefully accepted.