Configure Bazel mirrors for JAX OSS builds.
This change configures Bazel to use a `downloader_config` that redirects GitHub downloads to official mirrors (mirror.bazel.build and mirror.tensorflow.org).
Mirroring dependencies serves three main purposes:
1. Reliability: Reduces build failures caused by GitHub intermittent
connectivity issues or downtime.
2. Performance: Leverages high-availability mirrors for core Bazel rules
and ML-ecosystem dependencies (e.g., gRPC, Abseil).
3. Rate Limiting: Decreases the footprint of direct calls to GitHub APIs/CDN
from CI environments like GitHub Actions.
The configuration includes a fallback to the original GitHub URLs to ensure the build remains functional for any dependencies not yet cached by the mirrors.
PiperOrigin-RevId: 910397054
