xDS k8s tests: fix a line in the url_map test script

The line copied from the Python script was hardcoded and not
handled by a variable, so I did not change it on the first pass.

packages/grpc-js-xds/scripts/xds_k8s_url_map.sh

@@ -40,7 +40,7 @@ build_test_app_docker_images() {
 
   pushd "${SRC_DIR}"
   docker build \
-    -f src/python/grpcio_tests/tests_py3_only/interop/Dockerfile.client \
+    -f ${BUILD_APP_PATH} \
     -t "${CLIENT_IMAGE_NAME}:${GIT_COMMIT}" \
     .