Add BOT to env variables to avoid extra analytics counts in the dashboards.
Here are the checks: https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/base/bot_detector.dart#L46

Change-Id: I161aa7c86b335340d51feabbae87a9c7c4ab89c5
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/2922
Reviewed-by: Tong Wu <wutong@google.com>