Bazel mirror of bazel-toolchains 3.2.0 broken link

Hi, I'm seeing warnings building common projects that have the `bazel-toolchains` as a dependency because the following mirror.bazel.build URL is 404ing.

https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/3.2.0.tar.gz

An example of one project whose bazel build process is displaying warnings about this is the @google Tink project:
https://github.com/google/tink/blob/23ce810c979b1105fe6a657a5667a7275764b411/tink_base_deps.bzl#L69-L79

This is the error I see:
```
$ git clone https://github.com/google/tink
$ cd tink/tools/tinkey
$ bazel build tinkey
WARNING: Download from https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/3.2.0.tar.gz failed: class com.google.devtools.build.lib.bazel.repository.downloader.UnrecoverableHttpException GET returned 404 Not Found
...
```

According to the releases page the bazel-toolchains (link below) is supposed to be available at two URLs, one being the mirror, but it seems the mirror is missing version 3.2.0.

https://releases.bazel.build/bazel-toolchains.html

Is the mirror being maintained?

Should we continue to use the mirror as a dependency location?

Start by checking the reported mirror.bazel.build URL and the bazel-toolchains entry on releases.bazel.build, then compare how the dependency is referenced in Tink's tink_base_deps.bzl at lines 69-79. Done means determining whether version 3.2.0 should be present on the mirror and documenting or addressing the supported dependency location.

Written by the indexing model from the issue text.