FR: Specifying fallback location for downloading Bazel binary

Reading https://github.com/bazelbuild/bazelisk?tab=readme-ov-file#where-does-bazelisk-get-bazel-from, it sounds like I can configure a single source location for downloading Bazel.

We want to use the public release of Bazel, but we are trying to get our Bazel builds to have a backup mirror of 3P deps in case of unforseen upstream issues. In Bazel proper, you can achieve this by using `--download_config`, but to bootstrap the build, seems Bazelisk needs to rely on a single source.

This FR is to add a mirror/backup option if the regular source is not reachable.

Start with the README section on where Bazelisk gets Bazel from and compare its single-source behavior with Bazel's --download_config option. Define how a primary source and fallback mirror should be selected and what should happen when the regular source is unreachable; the issue does not name implementation files or tests.

Written by the indexing model from the issue text.