Re-enable PEP8 check in Win CI build (#4075)
* Add flake8 to Win CI build so it's re-enabled. It was in the static analysis build that is currently disabled so checks are not running.
Fix build.py to be compliant again.
Add prefix to flake8 output so it's (hopefully) easier to identify the errors in build output.
* Add to all builds in Windows CPU CI so they all fail quickly if there's an issue.
