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`.

Start with .bazelrc and bazel/downloader.cfg, then run `build --experimental_downloader_config=bazel/downloader.cfg` in the macOS Bazel Kokoro CI context to reproduce the 404 errors. Trace the failing mirror URLs and verify the downloader configuration. Done means the root cause is understood and the experimental downloader setting is uncommented and functional in .bazelrc.

Written by the indexing model from the issue text.