You can't. You could create a floor plane and raise that.
You can switch over to the Max sky instead.
( it is actually an OSL shader in cloak )
Zap added the possibility to make it move up and down to fake the horizon so it get visually pleasing rather than correct.
I used to sort these things with a curve on IOR on a plane behind scene so I could gradually bend the rays without having any seams.
Since then I wrote several OSL sky shaders, some considered, unnatural ( voice like Palpertine )
The feature got added after Aaron Ross suggested it would be practical.
This as Lee mentions is only possible on the map found in max Environment shader category, not the Native Arnold sky.
You insert the map in the Arnold render panels background, and you need to turn it 90 deg since there is a glitch when doing so.