pw_env_setup: Add durations to steps

Example output:

  Setting up CIPD package manager...done (12.9s)
  Setting up Python environment.....done (1m18.3s)
  Setting up Host tools.............done (0.1s)

Change-Id: I987b4cfa3cd37974f401d36aeca3d7d6e4f31d30
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49760
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Keir Mierle <keir@google.com>