Mark linux-focal-py3.8-gcc7 / test (distributed) as unstable temporarily (#95002)
This has become flaky recently (5.11% > 5% threshold) https://hud.pytorch.org/reliability/pytorch/pytorch?jobName=pull%20%2F%20linux-focal-py3.8-gcc7%20%2F%20test%20(distributed), moving it to unstable makes sense because the more important CUDA distributed jobs are still run in trunk. The issue is being investigated in https://github.com/pytorch/pytorch/issues/94954
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95002
Approved by: https://github.com/ZainRizvi