Set builder tag on test shard swarming tasks.

I think this is not interpreted by anything. Mainly interesting for filtering in the Swarming Task List UI.

Change-Id: I5054d6f935ef03c4534c81a23d6ab5576a9040d3
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/514600
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>