Investigate Re-enabling Experimental Bazel Downloader Config

In pull request #15545, the experimental Bazel downloader configuration was commented out in `.bazelrc` to resolve build failures on the macOS bazel Kokoro CI. This was necessary because the downloader was causing 404 errors during the build.

As noted by @scotthart in https://github.com/googleapis/google-cloud-cpp/pull/15545#discussion_r2352861699, using alternate mirrors via a downloader config (`bazel/downloader.cfg`) is beneficial for build stability, as it helps reduce flakiness by providing backups if canonical sources are temporarily unavailable.

This issue is to track the work required to:

1. Investigate the cause of the 404 errors when using `bazel/downloader.cfg`.
2. Update the `bazel/downloader.cfg` file and any related configurations to ensure the mirror URLs are correct and accessible.
3. Re-enable the experimental downloader in `.bazelrc` once it's confirmed to be working reliably.

**Acceptance Criteria:**

* The root cause of the previous 404 errors is understood.
* The `bazel/downloader.cfg` is verified to work correctly.
* `build --experimental_downloader_config=bazel/downloader.cfg` is uncommented and functional in `.bazelrc`.

Bắt đầu với .bazelrc và bazel/downloader.cfg, sau đó chạy `build --experimental_downloader_config=bazel/downloader.cfg` trong ngữ cảnh CI macOS Bazel Kokoro để tái hiện các lỗi 404. Truy vết các URL mirror gây lỗi và xác minh cấu hình downloader. Hoàn tất khi đã hiểu nguyên nhân gốc rễ và thiết lập experimental downloader được bỏ chú thích, hoạt động trong .bazelrc.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.