[Interop] Re-add SSA env var flip until safe to remove (#34460)

tools/run_tests/xds_k8s_test_driver/kubernetes-manifests/client-secure.deployment.yaml

@@ -51,6 +51,8 @@ spec:
             value: "true"
           - name: GRPC_XDS_EXPERIMENTAL_V3_SUPPORT
             value: "true"
+          - name: GRPC_EXPERIMENTAL_XDS_ENABLE_OVERRIDE_HOST
+            value: "true"
         volumeMounts:
           - mountPath: /tmp/grpc-xds/
             name: grpc-td-conf

tools/run_tests/xds_k8s_test_driver/kubernetes-manifests/client.deployment.yaml

@@ -55,6 +55,8 @@ spec:
             value: "true"
           - name: GRPC_EXPERIMENTAL_XDS_CUSTOM_LB_CONFIG
             value: "true"
+          - name: GRPC_EXPERIMENTAL_XDS_ENABLE_OVERRIDE_HOST
+            value: "true"
         volumeMounts:
           - mountPath: /tmp/grpc-xds/
             name: grpc-td-conf

tools/run_tests/xds_k8s_test_driver/kubernetes-manifests/server-secure.deployment.yaml

@@ -54,6 +54,8 @@ spec:
             value: "true"
           - name: GRPC_XDS_EXPERIMENTAL_RBAC
             value: "true"
+          - name: GRPC_EXPERIMENTAL_XDS_ENABLE_OVERRIDE_HOST
+            value: "true"
         volumeMounts:
           - mountPath: /tmp/grpc-xds/
             name: grpc-td-conf

tools/run_tests/xds_k8s_test_driver/kubernetes-manifests/server.deployment.yaml

@@ -47,6 +47,8 @@ spec:
             value: "/tmp/grpc-xds/td-grpc-bootstrap.json"
           - name: GRPC_XDS_EXPERIMENTAL_V3_SUPPORT
             value: "true"
+          - name: GRPC_EXPERIMENTAL_XDS_ENABLE_OVERRIDE_HOST
+            value: "true"
         volumeMounts:
           - mountPath: /tmp/grpc-xds/
             name: grpc-td-conf