Add a script to mirror 404 artifacts for building Bazel (#2386)

- Fetch logs from all jobs in the latest Bazel postsubmit on the master
branch.
- Parse logs to find 404 mirror artifacts by `Download from
(https?://mirror\.bazel\.build\S+)\s+failed: class
java.io.FileNotFoundException GET returned 404 Not Found`.
- Mirror those artifacts to bazel-mirror, it will NOT override existing
artifacts.