[ci] add retry to clean before checkout step
This was failing intermittently (e.g.
https://github.com/pytorch/pytorch/runs/6556614531?check_suite_focus=true).
I suspect it is due to this:
https://unix.stackexchange.com/questions/506319/why-am-i-getting-directory-not-empty-with-rm-rf
So adding a retry.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78098
Approved by: https://github.com/malfet, https://github.com/seemethere
