Update bazel-toolchains mirror URL
Update bazel-toolchains mirror URL

### What
Change the bazel-toolchains mirror URL to use the release archive format
instead of the commit archive format.

### Why
According to @smukherj1 from the bazel project in bazelbuild/bazel-toolchains#891 the GitHub commit archive URL is no longer mirrored by mirror.bazel.build. Instead the GitHub release archive URL is mirrored. This incorrect URL is causing bazel to output the following warning during build:

```
$ git clone https://github.com/google/tink
$ git checkout 23ce810c979b1105fe6a657a5667a7275764b411
$ 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
...
```

google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink