Fix broken gcloud auth plugin package name in qa/Dockerfile

Context

google-cloud-sdk-gke-gcloud-auth-plugin no longer exists in the Google Cloud apt repo — apt-get install fails with a missing package error. The replacement is google-cloud-cli-gke-gcloud-auth-plugin. This broke build-qa-image on 2026-08-27, blocking ~20 master pipelines. Tracked in gitlab-org/quality/analytics/ci-health-incidents#1201 (closed).

What's in this MR?

Renames the package in qa/Dockerfile to the current name.

Verification

Confirmed directly against the apt repo — google-cloud-cli-gke-gcloud-auth-plugin is listed, google-cloud-sdk-gke-gcloud-auth-plugin is not. The fix will be confirmed by a passing build-qa-image job on master after merge.

Related

• Incident: gitlab-org/quality/analytics/ci-health-incidents#1201 (closed)