[dart_ci] Install dart-sdk from fixed version and tag CIPD package as latest

Change-Id: Id242512b347f5b9852d26ef5b3a4ee0e8feae8ad
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/203880
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Karl Klose <karlklose@google.com>
3 files changed