Disable XRT in default CI jobs (#5135)
* PJRT_ONLY -> DISABLE_XRT
* Add XRT CPU test workflow
* Disable XRT in coverage
* Disable XRT in TPU CI
* Use `latest-xrt` in image tag
* Fix syntax
* Only run python tests for XRT
* Remove validation for `disable_xrt`
* Move autocast test with other python tests
* Fix path typo
