Provide a way to exclude specific target patterns from `build_targets`

[brandjon]

I'd like to exclude my docs/ directory from build_targets because it depends on an unreleased bazel version to build (i.e., regenerate the documentation) successfully. Until that newer bazel is out I'm just relying on the custom bazel on my workstation and there's nothing for CI to check.

At the moment this means changing my presubmit.yml so instead of build_targets being ... it enumerates every top-level directory besides docs/, which is error-prone.

[jin]

You can exclude targets with

build_targets:
  - "--"
  - "//..."
  - "-//docs/..."