make sure dispatch test works on windows (#36729)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36729
setenv not available on windows
Test Plan: CI green in ovrsource
Reviewed By: stepancheg
Differential Revision: D21067835
fbshipit-source-id: ddbc3285ef88f123dc6a200b661c48cfafc6bf00
