Use action user as the trigger user of schedules (#30581) (#30609)

Use action user as the trigger user of schedules (#30581) (#30609)

Backport #30581 by @yp05327

Follow https://github.com/go-gitea/gitea/pull/30357

When user push to default branch, the schedule trigger user will be the
user.
When disable then enable action units in settings, the schedule trigger
user will be action user.
When repo is a mirror, the schedule trigger user will be action user. (
before it will return error, fixed by #30357)

As scheduled job is a cron, the trigger user should be action user from
Gitea, not a real user.

Co-authored-by: yp05327 <576951401@qq.com>