Instead, if a fixed-output derivation produces has an incorrect output
hash, we now unconditionally move the outputs to the path
corresponding with the actual hash and register it as valid. Thus,
after correcting the hash in the Nix expression (e.g. in a fetchurl
call), the fixed-output derivation doesn't have to be built again.

It would still be good to have a command for reporting the actual hash
of a fixed-output derivation (instead of throwing an error), but
"nix-build --hash" didn't do that.