Fix deadlock in SubstitutionGoal

We were relying on SubstitutionGoal's destructor releasing the lock,
but if a goal is a top-level goal, the destructor won't run in a
timely manner since its reference count won't drop to zero.  So
release it explicitly.

Fixes #178.

src/libstore/build.cc

@@ -2852,6 +2852,7 @@ void SubstitutionGoal::finished()
     worker.store.registerValidPath(info2);
 
     outputLock->setDeletion(true);
+    outputLock.reset();
 
     worker.store.markContentsGood(storePath);