* Generate local metadata even when not publishing.

For SLSA compliance we need to separate the fetch, compile and upload
steps of release artifacts. Translating this to the packaging workflows
the fetch step will checkout the prepare_package script at main ToT, the
compile step generate the bundle archives and the recipes will upload
the artifact bundles as part of the upload stage.

This change adds functionality to generate both the release bundle and
the updated metadata file in a way that both files can be uploaded as
part of the upload stage.

Bug: https://github.com/flutter/flutter/issues/115487

* Address comments.

* Update tests.