I think the LocalStore constructor can't open the sqlite db in read-only mode, when
the 'copy-from-other-stores.pl' script runs "nix-store --check-validity" there.

The LocalStore::openDB in any case opens the sqlite database readonly, for what I saw (19d9762).

I'd really like to be able to use NIX_OTHER_STORES without being required writing access, and with the security of not writing anything to those.