Don't run CI if backport PRs have the label conflicts

[christophfroehlich]

as per request of @saikishor

maybe with
if ${{ ! contains(toJson(github.event.pull_request.labels.*.name), 'conflicts') }}

https://stackoverflow.com/questions/71481086/is-pull-request-label-available-to-a-github-action
https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#jobsjob_idif

but I'm not sure if this event is retriggered if a new commit is added

[saikishor]

We can give it a shot when we have some time and see. This could be helpful.
When we have merge conflicts, we can have mergify add conflicts tag automatically too