xtriggers not sequence-specific

[hjoliver]

Describe the bug

Unlike task triggers, xtriggers are not being tied to the cycling sequence. So a task that has different xtriggers in different sequences will end up getting all the xtriggers every time.

REF: https://cylc.discourse.group/t/different-clock-triggers-for-different-cycles/120

Credit @trwhitcomb for finding this 👍

Release version(s) and/or repository branch(es) affected?

Up to and including 7.8.3

Pull requests welcome!
This is an Open Source project - please consider contributing a bug fix
yourself (please read CONTRIBUTING.md before starting any work though).