Delete "-b" flag from pip install command (#48722)
Summary:
"--build <dir>" flag has been deprecated for a while and finally removed in pip-20.3
Before this PR is applied, every change to docker images would result in ONNX failures
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48722
Reviewed By: janeyx99
Differential Revision: D25274020
Pulled By: malfet
fbshipit-source-id: 9e0f9daba58ceeec5474d649d1b22bfeca91d7bc
