[bazel] Add "label" part to "BazelTest" task schema.

Context:
https://skia-review.googlesource.com/c/skia/+/694625/18/infra/bots/recipe_modules/builder_name_schema/builder_name_schema.json#23.

Bug: skia:14227
Change-Id: I6f985e4027abc84a23cc4ee8cd1dbe98e41e13c2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/700224
Commit-Queue: Leandro Lovisolo <lovisolo@google.com>
Auto-Submit: Leandro Lovisolo <lovisolo@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
8 files changed