[GHA] Clean after checkout (#70337)
Summary:
Github's checkout action sometimes leaves untracked files in the repo
Remedy it by running `git clean -fxd`, which should nuke them all
Tentative fix for https://github.com/pytorch/pytorch/issues/70097
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70337
Reviewed By: suo
Differential Revision: D33289189
Pulled By: malfet
fbshipit-source-id: 16e3ebe7a61fda1648189c78bdf1b1185247037a
