https://mirror.bazel.build is not populated for toolchain 4.1.0

[jtattermusch]

Context: grpc/grpc#27410 (comment)

Looks like the bazel download mirror hasn't been populated with the 4.1.0 release.

I'm getting this warning:

WARNING: Download from https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/4.1.0.tar.gz failed: class com.google.devtools.build.lib.bazel.repository.downloader.UnrecoverableHttpException GET returned 404 Not Found

[vihangm]

Any updates here? This is still missing.