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

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.