Fix cwd error in print_step.
Fix cwd error in print_step.

We should be more explicit about the guarantees of our data structures.
Probably time to make step a real thing.

BUG=571206
TBR=iannucci@chromium.org

Review URL: https://codereview.chromium.org/1558753002

luci/recipes-py,luci/recipes-py