Fix windows msbuild bug (#18748)
Summary:
Fix the bug introduced by #18681 where an undefined variable was being used to limit max cpu count when building for Windows without Ninja.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18748
Differential Revision: D14733209
Pulled By: soumith
fbshipit-source-id: 52fc0dd4dde99da75a6956b63f02da2e647eed4f
