Have them originally evolve somewhere else and move there. After all humans evolved for the savannah and use technology to live in the arctic.
If they are intelligent they should be using fire and clothing anyway, and tech can make up for a lot of shortcomings. maybe they live in the arctic NOW because other fast breeding races pushed them out of more hospitable areas, like how we used to think humans had done to Neanderthals.
There is also no reason your lizard people cannot be endothermic there are plenty of lizard looking things in history that were endothermic, archosaurs and early mammals are good examples.
If you need them to be ectotherms then you have some problems. They are going to be inactive most of the year and hibernate the rest, this is rather unlikely for an intelligent creature, brains eat a lot of calories (1/3 of all our calories) and need them all the time and an ectoderm may not be able to support this.
Creatures with a slow metabolism are unlikely to evolve human like intelligence, they just can't spare the calories.