Add configurable label to PR if backport fails

[paulgessinger]

RIght now the action comments if the backport merge failed for some reason. It would be super helpful if it could (optionally) also attach a label to a failed PR, so it's easier to filter for them and manually backport.

[Pierre-Sassoulas]

Thank you for creating this github actions.

I was going to open the exact same issue. If the cherry-pick has a conflict and the backport fail we should be able to add a label. I'd like to add "Needs backport" but if it's not configurable and need to be another name we can make it work too :)

Let me know if I can help in any way.