ci: use context.issue.number for removing ok-to-test

`github.event.pull_request.number` does not seem to be valid as a script
object/variable.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

.github/workflows/pull-request-commentor.yaml

@@ -119,7 +119,7 @@ jobs:
         with:
           script: |
             github.rest.issues.removeLabel({
-              issue_number: github.event.pull_request.number,
+              issue_number: context.issue.number,
               owner: context.repo.owner,
               repo: context.repo.repo,
               name: ["ok-to-test"]