Add support for configuring the base storage URL for Flutter's
artifacts. If FLUTTER_STORAGE_BASE_URL is set, use it instead of
storage.googleapis.com.
The pub server can be overridden by setting PUB_HOSTED_URL.
I'm not really sure how to test this without a reference test, since
eventually we want to move this to a font feature and thus the obvious
test (looking for the Transform widget) isn't really valid.
