The envtest recipe now works for Windows

Changes required to get the envtest recipe to work on Windows.
(Mostly a reland of pwrev/6740.)

Simplified way of setting PW_ROOT since then because of pwrev/6783.

Tested with led:

$ led get-builder luci.pigweed.ci:pigweed-windows-envtest \
    | led edit-recipe-bundle \
    | led launch
...
LUCI UI: https://ci.chromium.org/swarming/task/4a82d9f67accc310?server=chrome-swarming.appspot.com

$ led get-builder luci.pigweed.ci:pigweed-linux-envtest \
    | led edit-recipe-bundle \
    | led launch
...
LUCI UI: https://ci.chromium.org/swarming/task/4a82da0ab342c810?server=chrome-swarming.appspot.com

Bug: 92
Change-Id: I9a021ce2d7c7040ad37e088ad8ff7d619dc1f573
4 files changed
tree: 15865a21c1c4806666ad1cf5432d257c137de611
  1. infra/
  2. recipe_modules/
  3. recipes/
  4. .gitignore
  5. .style.yapf
  6. recipes.py