Expose building an archive as an API

I have a wrapper for building executables that I would like to move from pex to shiv. However, the API to build executables is not exposed. It seems like starting around the `with Temporary....` would be a good place to split up and call an API function.

Would that make sense as a PR?

Start around the `with Temporary...` section identified in the issue and trace how the CLI currently builds executables. Determine the public API boundary and how callers would invoke archive building, then verify that the existing command-line behavior still works. Done means archive construction is exposed for external wrappers without relying on internal CLI flow.

Written by the indexing model from the issue text.