specify pool image to Ubuntu-20.04 for SendEmail job (#20128)
specify pool image to Ubuntu-20.04 for SendEmail job (#20128)


yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli