[bazel] Rebuild bazel_workspace when necessary

... in addition to just build.ninja. This ensures all bazel_workspace
dependencies, most importantly core and platform SDKs, are up-to-date.

Change-Id: Ib42fde3479bf29f2349029c482e1ed281cf9cfff
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/778502
Reviewed-by: Shai Barack <shayba@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Fuchsia-Auto-Submit: Jay Zhuang <jayzhuang@google.com>