fr/idea: allow projects to include bazelrc files for the downstream/bazel-at-head pipelines

While trying to futz with some bazelrc files, I thought of an idea that might be useful for the bazel-at-head and the "run all the downstream projects" pipelines.

Basically, when those pipelines run, they conditionally include additional bazelrc config files or config names. e.g.g
* Add `--config=bazel_head` to the flags
* Check if `bazel_head.bazelrc` exists, if so, add `--bazelrc=bazel_head.bazelrc`

In any case, the basic idea is Bazel devs and project owners have a way to set necessary flags for the upcoming version of bazel and get cleaner pipeline results. e.g they run the pipeline, identify failures, update the projects to set necessary flags, and the next pipeline run will have cleaner results.

Anyways, just a thought I had and wanted to let relevant people know.

related bazel fr: https://github.com/bazelbuild/bazel/issues/24043