By addding timer trigger events directly to the pipeline event queues
(added in I6b4b77a82aac6d2c3441ace9f64dc9b4a80c5856) we no longer use
the `addTriggerEvent()` method of the scheduler which was setting the
`arrived_at_scheduler_timestamp` of the event.
This will lead to an exception when reporting pipeline stats for the
added item:
ERROR zuul.Pipeline.tenant.periodic_5_min: Exception reporting pipeline stats
Traceback (most recent call last):
File "/opt/zuul/lib/python3.10/site-packages/zuul/manager/__init__.py", line 2208, in reportStats
processing = (now - arrived) * 1000
TypeError: unsupported operand type(s) for -: 'float' and 'NoneType'
This changes sets the missing timestamp when creating the event in the
timer driver.
Change-Id: Ic6fe39637935405dd0df6c9bac2e3bf313656a63