Use bazelisk instead of specifying bazel version manually. (#33036)
Summary:
Bazelisk automatically reads `.bazelversion` file and install the required version of Bazel. This saves us from updating CI script everytime we need a Bazel upgrade.
Use clang-8 for consistency with pytorch/xla repo.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33036
Differential Revision: D19820819
Pulled By: ailzhang
fbshipit-source-id: 1560ec225cd037a811769a509a704b0df77ea183
