Document is inconsistent with code, bazel.dev use clang instead of gcc


*Description*:

Document is inconsistent with code

in ci/README file,
>bazel.dev — build Envoy static binary and run tests under -c fastbuild with gcc.

but in do_ci.sh
```sh
elif [[ "$1" == "bazel.dev" ]]; then
  setup_clang_toolchain
```
