Build-test-hello-world sample will be broken if enable the cache

[huanwu]

Simple update https://github.com/Azure-Samples/acr-tasks/blob/master/build-test-hello-world.yaml and add cache:enabled after each build.

Run it and it will return the following error "Unable to find image 'hello-world-test:latest' locally
docker: Error response from daemon: pull access denied for hello-world-test, repository does not exist or may require 'docker login': denied: requested access to the resource is denied.
See 'docker run --help'.
2019/10/08 21:52:38 Container failed during run: func-tests. No retries remaining.
failed to run step ID: func-tests: exit status 125
Run ID: ycg failed after 12s. Error: failed during run, err: exit status 1
Run failed
"