Please support linting BUILD and BUILD.bazel files in the same repository

Currently the CI runner script looks for a file named `BUILD.bazel`, and if it finds one, ignores all files named `BUILD`. This doesn't work well for repos with both names -- for example, mostly using `BUILD` but naming the top-level file `BUILD.bazel` to avoid conflicts with a `build/` directory on case-insensitive filesystems.

It would be nice if this behavior could be overridden in `.bazelci/presubmit.yml`.