[beta] Add support for the beta channel to the recipes

https://github.com/dart-lang/sdk/issues/40987

Change-Id: Ibdf83a0bdf21d5c3d31e76a5350151963bc42e3e
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/138822
Reviewed-by: Karl Klose <karlklose@google.com>
2 files changed