`ready-to-merge` label is incorrectly applied even though `needs-rebase` has been set

See screenshot below:

![image](https://user-images.githubusercontent.com/3885308/81254128-16ba2b80-8fdf-11ea-8eed-8d5f1884817d.png)

PR from screenshot: https://github.com/cilium/cilium/pull/11328