Fix failing release GitHub action (#460)

Code must first be checked out.

GitHub action https://github.com/rabbitmq/cluster-operator/runs/1380561796?check_suite_focus=true failed with message:
level=fatal msg="open /github/workspace/hack/rabbitmq.yaml: no such file or directory"

Author: ansd

.github/workflows/release.yml

@@ -8,6 +8,8 @@ jobs:
   kubectl_rabbitmq:
     runs-on: ubuntu-latest
     steps:
+    - name: Checkout
+      uses: actions/checkout@main
     - name: Update new version in krew-index
       uses: rajatjindal/[email protected]
       with: