Trying to make sure it work with obscurers stores.

We downcast in a few places, this will be refactored to be better later.

Remote stores still override so the other end schedules.

Just a few small things needed fixing!

…uild

Thanks @regnat and @edolstra for catching this and comming up with the
solution.

They way I had generalized those is wrong, because local settings for
non-local stores is confusing default. And due to the nature of C++
inheritance, fixing the defaults is more annoying than it should be.
Additionally, I thought we might just drop the check in the substitution
logic since `Store::addToStore` is now streaming, but @regnat rightfully
pointed out that as it downloads dependencies first, that would still be
too late, and also waste effort on possibly unneeded/unwanted
dependencies.

The simple and correct thing to do is just make a store method for the
boolean logic, keeping all the setting and key stuff the way it was
before. That new method is both used by `LocalStore::addToStore` and the
substitution goal check. Perhaps we might eventually make it fancier,
e.g. sending the ValidPathInfo to remote stores for them to validate,
but this is good enough for now.

Thanks!

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

Make `nix-build --store whatever` work