ci: twister: use workflow_call for prep job

Use workflow_call to allow for different type of nodes depending on
event.

On push, we do not need to use zephyr runners, GH runners are enough and
are much faster to deploy and start.

This resolves an issue where push jobs will have to be queued for a
longer time waiting for the prep step, once the prep step is done, we
will have to wait one more time in the queue for requested nodes.

This should speed up execution of push events in CI.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>