Use std::variant to enforce BuildResult invariants

There is now a clean separation between successful and failing build
results.

src/libstore/local-store.cc

@@ -997,7 +997,7 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
             }},
             {[&](const StorePath & path, const StorePath & parent) {
                 return BuildError(
-                    BuildResult::OutputRejected,
+                    BuildResult::Failure::OutputRejected,
                     "cycle detected in the references of '%s' from '%s'",
                     printStorePath(path),
                     printStorePath(parent));