[18.09 backport] Consider WINDOWS_BASE_IMAGE_TAG override when setting Windows base image for tests Upstream-commit: bb54c5bf2a3ab1c698fda6b3d67773d25755ae45 Component: engine
[18.09 backport] Consider WINDOWS_BASE_IMAGE_TAG override when setting Windows base image for tests Upstream-commit: bb54c5bf2a3ab1c698fda6b3d67773d25755ae45 Component: engine