Russia is well aware that if they use nuclear weapons it's over for them, will the rest of the world fall too is another question.
Either way I truly don't believe they will do it simply because that ends Russia in its current form, and I don't think we should allow them to be an aggressor just because they have nukes. Russia needs to be stopped and their saber rattling should rightly be ignored.
Feel free to @ me if you die in a nuclear apocalypse to prove me wrong. Till then let's keep enabling Ukraine to fight back and continue Russias massive embarrassment and degradation of power and capabilities.