update path in CI script to access ninja (#43236)
Summary:
This relaxes the assumption that test.sh will be run in the CI environment by the CI user.
CC ezyang xw285cornell sunway513
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43236
Reviewed By: colesbury
Differential Revision: D23205981
Pulled By: ezyang
fbshipit-source-id: 302743cb03c9e9c6bfcdd478a6cd920b536dc29b
