Make some more threads receive interrupts

Shouldn't hurt to do this. In particular, this should speed up
shutting down the PathSubstitutionGoal thread if it's copying from a
remote store.

src/libutil/thread-pool.cc

@@ -79,6 +79,8 @@ void ThreadPool::process()
 
 void ThreadPool::doWork(bool mainThread)
 {
+    ReceiveInterrupts receiveInterrupts;
+
     if (!mainThread)
         interruptCheck = [&]() { return (bool) quit; };