Rename mirror-404-artifacts-for-bazel to mirror-bazel-external-deps and use tools/mirror_external_deps.py (#2912)

Rename the `mirror-404-artifacts-for-bazel` pipeline to
`mirror-bazel-external-deps` ("Mirror Bazel External Deps") and update
it to check out `bazelbuild/bazel` and run
`./tools/mirror_external_deps.py --upload` to discover and upload
missing external repository URLs to `gs://bazel-mirror`, replacing
`buildkite/mirror_404_downloads.py`.