Set USE_GKE_GCLOUD_AUTH_PLUGIN for GKE deploy workflow

Modern gcloud CLI (>= 400) requires this env var so kubectl uses the
installed gke-gcloud-auth-plugin binary instead of the deprecated
application-default credentials path. Without it, kubectl commands
silently fail even after get-credentials succeeds.

.gitea/workflows/deploy-gke.yaml

@@ -22,6 +22,9 @@ jobs:
   deploy:
     runs-on: ubuntu-latest
 
+    env:
+      USE_GKE_GCLOUD_AUTH_PLUGIN: "True"
+
     steps:
       - name: Checkout
         uses: actions/checkout@v4