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`.