[devicelab] Propagate git_url and git_ref to subbuilds.

Also as the swarming url is disallowd on milo, replace it with the milo url.

Bug: https://github.com/flutter/flutter/issues/64346
Test: https://ci.chromium.org/swarming/task/4e2b2305f6db3110?server=chromium-swarm.appspot.com
Change-Id: I9cada20e5199f616165a70f757fc0b5007751660
Fixed: 64346
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/5920
Reviewed-by: Keyong Han <keyonghan@google.com>
Commit-Queue: Tong Wu <wutong@google.com>