Add fallback for airplane mode

What's the recommended solution for running Bazel without an internet connection? I vendored all my external deps, opened my laptop on the airplane ready to do some work, and discovered I couldn't run bazel because I only have bazelisk installed.

Is there a command-line flag for storing locally and running the most recently cached version? Or do I need to install bazel without bazelisk if I need offline access?

The issue names no file, test, or entry point. Start by tracing Bazelisk's command-line version selection and offline behavior, then define the expected fallback for a cached Bazel version; done should mean Bazelisk can run without internet access when a usable cached version exists.

Written by the indexing model from the issue text.