Find a better way to skip the CANCELLED error log while shutting down.

[pipecd-bot]

https://github.com/pipe-cd/pipe/blob/12396182979ad9287880d483f882b9c81ad519a5/pkg/app/piped/trigger/trigger.go#L209-L212

This was created by todo plugin since "TODO:" was found in https://github.com/pipe-cd/pipe/commit/12396182979ad9287880d483f882b9c81ad519a5 when #2777 was merged. cc: @nghialv.