CI: limit the on push triggers (#35574)

CI: limit the on push triggers (#35574)

Since now we have workflow_dispatch, we can trigger a workflow manually in case we don't have an open PR.

Also, remove the custom `ci skip` code; it's supported natively for some time now: https://github.blog/changelog/2021-02-08-github-actions-skip-pull-request-and-push-workflows-with-skip-ci/