ci: disable `last_rc` testing until `--repo_contents_cache` is resolved

```
ERROR: The repo contents cache [/builds/bazel/download_utils/.cache/bazel/repo/contents] is inside the workspace [/builds/bazel/download_utils]. This can cause spurious failures. Disable the repo contents cache with `--repo_contents_cache=`, or specify `--repo_contents_cache=<path outside the workspace>`.
```