Allow setting metadata-dir separately from output-dir

[ncoghlan]

The current CLI help provides no indication that the metadata for built archives and locally exported environments will be published to <output path>/__venvstacks__/ by default.

Adding a --metadata-dir option will both provide a place to document the default location, and allow the metadata to be placed somewhere else if desired (e.g outside the artifact folder, in a location that makes caching or publishing the manifests separately from the built archives straightforward).