Document how to run build that depend on external tools

... or if it's already documented, please link to that from the [front page](https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/README.md#faq).

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While trying to integrate with libraries that don't already build under Bazel it's rather common to end up with dependencies that bazel can't build ([e.g.](https://buildkite.com/bazel/bcsgh-bazel-rules/builds/36#92e5c0dc-c98b-475e-9136-a8cdc2b8c68b)). While the correct thing to do is bring that dependency inside the bazel build, that's not always possible (the library's build may intentionally assume the presence of a lib or tool and chose to offer no means to get it from somewhere else).

Which is all to say that, like it or not, running CI's with arbitrary extra preconditions imposed on the image/environment/platform is going to be an ongoing requirement basically forever and supporting that, while not something that needed to be easy, should be a **documented** process.

Start with buildkite/README.md, especially the FAQ section linked in the issue, and review the existing CI documentation for how builds are configured. Document the process for running builds with dependencies on external tools or environment prerequisites, or add a link to the existing explanation from the front page. Done means a contributor can find and follow the documented process.

Written by the indexing model from the issue text.