[ci] move XLA pin update to weekly (#84208)
- Create a `weekly` workflow and move XLA pin update to that
- Move the other two pin updates to the `nightly` workflow (instead of
having a special workflow just for them.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84208
Approved by: https://github.com/janeyx99, https://github.com/malfet, https://github.com/seemethere
