use xenial image for testing since trusty does not have python3.9

Author: mattip

azure-pipelines.yml

@@ -88,11 +88,13 @@ jobs:
           PLAT: "i686"
           MB_ML_VER: "2010"
           ENV_VARS_PATH: "env_vars_32.sh"
+          DOCKER_TEST_IMAGE: "multibuild/xenial_{PLAT}"
         py_3.9_64manylinux2010:
           MB_PYTHON_VERSION: "3.9"
           # Comment this out when the image grows a CPython3.9
           AZURE_PYTHON_VERSION: "3.8"
           MB_ML_VER: "2010"
+          DOCKER_TEST_IMAGE: "multibuild/xenial_{PLAT}"
 
         # manylinux1 wheels
         py_3.6_32manylinux1: