Correct the labeler GitHub action

[NicolasReyrolle]

The labeler workflow was not working. I have corrected it to make it work according to the v4. There were 2 errors:

• checkout is needed ("The configuration file (path: .github/labeler.yml) was not found locally, fetching via the api")
• the file format for the labeler.yml was not the good one. I have not been able to understand if that comes from v4, but I guess this is the case

Does not work on you repo (I guess that needs to be merged to master first), but that's good on my fork