fix: install auth plugin on gke environment tests (#1232)

* install auth plugin on gke environment tests

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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: daniel-sanche

.kokoro/environment_tests.sh

@@ -63,6 +63,9 @@ if [[ "${ENVIRONMENT}" == "kubernetes" ]]; then
   mkdir -p ~/.local/bin
   mv ./kubectl ~/.local/bin
   export PATH=$PATH:~/.local/bin
+  # install auth plugin
+  gcloud components install gke-gcloud-auth-plugin -q
+  export USE_GKE_GCLOUD_AUTH_PLUGIN=True
 fi
 
 # create a unique id for this run