avoid "set -x" on cluster-creator-login/cluster-creator-login.sh script

I realized while reviewing !2308 (merged) that the cluster-creator-login/cluster-creator-login.sh script is leaking kubeconfig Secrets in Job logs (easily observed with kubectl logs -n kube-job cluster-creator-login-flux-system-xxxxx).

This is a small security issue (access to those logs is today subject to the same restrictions as the access to the Secret that is leaked).

The issue was introduced in !2003 (merged) -- /cc @baburciu @loic.nicolle and myself -- we wrote or reviewed that