Pixar and Disney mostly use Linux (as do most of the VFX / CG studios).

Artists can use Windows / OS X in some cases, but given the main apps they use (Maya, Mari, Modo, Katana, Nuke) all run on Linux, there's no point really, and using Linux everywhere makes admin much easier and more consistent.

There are some apps that are used which are Windows-only (ZBrush), and so certain artists/modellers who need to use it will use that, but these are generally the exception.