[BE] Remove pip and conda installation in Linux build workflow (#87256)
All the dependencies should come from the Docker container already. This only updates Linux build workflow, Linux test workflow comes later in a separate PR.
The `opt-einsum` package that was installed as part of PyTorch wheel has already been installed in the Docker container [requirements-ci.txt](https://github.com/pytorch/pytorch/blob/master/.circleci/docker/requirements-ci.txt#L127)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87256
Approved by: https://github.com/malfet
