dev/ci/cluster: deploy to unique BUILDKITE_JOB_ID namespace

Created by: bobheadxi

Using just BUILDKITE_BUILD_NUMBER means that namespaces will be used across re-runs of the cluster test job. If you attempt a re-run before the previous cluster has had time to properly spin down, this can cause a re-run to fail, e.g. https://buildkite.com/sourcegraph/qa/builds/5959#31bc1366-5e4a-4e8d-ac31-9bdf4d323b6a:

Error from server (Forbidden): error when creating "base/worker/worker.Deployment.yaml": deployments.apps "worker" is forbidden: unable to create new content in namespace cluster-ci-5959 because it is being terminated

It seems BUILDKITE_JOB_ID is unique across retries so we use that in cluster namespace as well.