Investigate Re-enabling Experimental Bazel Downloader Config

[jinseopkim0]

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.