[cloudbuild] support for steps[*].dir

[bradrydzewski]

https://cloud.google.com/build/docs/build-config-file-schema#dir

Use the dir field in a build step to set a working directory to use when running the step's container. If you set the dir field in the build step, the working directory is set to /workspace/

. If this value is a relative path, it is relative to the build's working directory. If this value is absolute, it may be outside the build's working directory, in which case the contents of the path may not be persisted across build step executions (unless a volume for that path is specified).

steps:
- name: 'gcr.io/cloud-builders/go'
 args: ['install', '.']
 env: ['PROJECT_ROOT=hello']
 dir: 'examples/hello_world'