.github: Migrate linux-xenial-py3.6-gcc7 to GHA (#67072)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/66888
cc seemethere
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67072
Reviewed By: seemethere
Differential Revision: D31900833
Pulled By: zhaoalex
fbshipit-source-id: 93f8995611169d991f90e07e8c13e08182969577
