Run self-assign and pr-updates only in create event

[Abacn]

mitigate #28909 . the number of github action workflow is limited by github, even we have self-hosted runner setup, we still rely github's resource to queue jobs.

Every time codecov updates its comment, these two actions get queued. This change would further reduce the number of queued actions by issue_comment trigger.

Please add a meaningful description for your change here

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[Abacn]

R: @damccorm

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control