workflows/labels: fix approved-by-maintainer label

Due to a type mismatch, maintainer approvals were never counted as such.
The API returns integers for the user IDs, but the JSON file has strings
as object keys.

.github/workflows/labels.yml

@@ -166,7 +166,7 @@ jobs:
 
                   const maintainers = new Set(Object.keys(
                     JSON.parse(await readFile(`${pull_request.number}/maintainers.json`, 'utf-8'))
-                  ))
+                  ).map(m => Number.parseInt(m, 10)))
 
                   // And the labels that should be there
                   const after = JSON.parse(await readFile(`${pull_request.number}/changed-paths.json`, 'utf-8')).labels