daemon: Allow builds to be repeated.

This makes it easy to detect non-deterministic builds.

* nix/libstore/build.cc (DerivationGoal): Remove 'InodesSeen'; add
'curRound', 'nrRound', and 'prevInfos'.
(DerivationGoal::inputsRealised): Initialize 'nrRound'.
(NotDeterministic): New error type.
(DerivationGoal::buildDone): Check whether we need to repeat.
(DerivationGoal::startBuilder): Adjust message.
(DerivationGoal::registerOutputs): Check whether we get the same result.
* nix/libstore/globals.cc (Settings::get(const string & name, int def)):
New method.
* nix/libstore/globals.hh (Settings): Add it.
* nix/libstore/store-api.hh (ValidPathInfo): Add operator ==.
* nix/nix-daemon/nix-daemon.cc (performOp): Allow "build-repeat" for
"untrusted" users.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>