Test rebuild trigger

[goodov]

TODO for a rebuild trigger via label to work:

1. add a separate workflow that will act on label add.
2. this separate workflow should trigger all required jobs via workflow_dispatch.

I haven't found a way to simulate "pull_request" event to trigger related jobs as if Github triggered them in a normal way.

the workflow that builds/tests stuff cannot act on label add on its own, because any added label will invalidate the last job state.