change label check method

.github/workflows/pr_ok_to_test.yaml

@@ -24,6 +24,7 @@ jobs:
   # Runs before all other jobs
   # builds the minikube binaries
   build_minikube:
+    if: ${{ contains(github.event.pull_request.labels.*.name, 'ok-to-test') }}
     runs-on: ubuntu-18.04
     steps:
       - name: Check ok-to-test label