Show Flake8 errors in GitHub CI again (#46990)

Summary:
Fixes #46985.

Can someone comment on whether the "Run flake8" step should fail if `flake8` produces errors? This PR makes sure the errors are still shown, but [the job linked from the issue](https://github.com/pytorch/pytorch/runs/1320258832) also shows that the failure of that step seems to have caused the "Add annotations" step not to run.

Is this what we want, or should I instead revert back to the `--exit-zero` behavior (in this case by just removing the `-o pipefail` from this PR) that we had before #46740? And if the latter, then (how) should I modify this `flake8-py3` job to make sure it fails when `flake8` fails (assuming it didn't already do that?)

Pull Request resolved: #46990

Reviewed By: VitalyFedyunin

Differential Revision: D24593573

Pulled By: samestep

fbshipit-source-id: 361392846de9fadda1c87d2046cf8d26861524ca

.github/workflows/lint.yml

@@ -69,11 +69,10 @@ jobs:
         id: get_pr_tip
       - name: Run flake8
         run: |
-          set -eux
+          set -eux -o pipefail
           pip install flake8==3.8.2 flake8-bugbear flake8-comprehensions flake8-executable flake8-pyi==20.5.0 mccabe pycodestyle==2.6.0 pyflakes==2.2.0
           flake8 --version
-          flake8 > ${GITHUB_WORKSPACE}/flake8-output.txt
-          cat ${GITHUB_WORKSPACE}/flake8-output.txt
+          flake8 | tee ${GITHUB_WORKSPACE}/flake8-output.txt
       - name: Add annotations
         uses: pytorch/add-annotations-github-action@master
         with: