Network cache size is fixed by a constant: https://github.com/metabrainz/picard/blob/0a36d1e0d5494cdc046b407854f0b8b3c0a8716a/picard/const/__init__.py#L52

Make it user-configurable, defaulting to this constant.

It was suggested in https://github.com/metabrainz/picard/pull/2393 though the issue it is supposed to fix is unclear, and the approach using an env var isn't the best for Picard.