borg2: check --verify-data --find-lost-archives can be made faster

`borg check` with `--verify-data` **and** `--find-lost-archives`:

In `verify_data`, we do a full pass over all repo objects to verify their cryptographic authenticity. We could build a list of object IDs of archive metadata objects as a side effect there.

If we have that, we could accelerate `rebuild_archives_directory` using that list instead of iterating and reading all objects again.

Note: `--find-lost-archives` is used rarely.