[beta] Make the docker recipe channel agnostic

* Use the latest stable Dart SDK CIPD package to run Dart scripts.
* Remove the build.git dependency.

https://github.com/dart-lang/sdk/issues/40991

Change-Id: I3261a6a30d83d476f3ecb3d7a184835a8e62a42d
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/142001
Reviewed-by: William Hesse <whesse@google.com>
4 files changed