Add support for source fetching + building

Right now bazelisk fetches binary distributions via github releases, I'm curious if adding support for source based fetching + building is something being considered or would be okay to contribute?

Perhaps some extra syntax in `.bazelversion` that instructs bazelisk to fetch a zip of some given sha/tag and build a release of bazel locally, and use that as the underlying bazel binary.

Start by reviewing Bazelisk's existing GitHub-release fetching flow and how .bazelversion is parsed. Define the source reference syntax, archive fetching, local Bazel build process, and resulting binary selection before implementation. Done means a documented source-based path works alongside binary fetching, with tests covering the new syntax and build flow.

Written by the indexing model from the issue text.