daemon: fix hang on SSH disconnect during remote builds

When an SSH connection dies during a remote build, MonitorFdHup correctly
detects the disconnect and calls triggerInterrupt(). However, without
ReceiveInterrupts instantiated, no SIGUSR1 is sent to interrupt the
blocking read() syscall. This causes the daemon to hang indefinitely
while holding file locks, blocking subsequent builds.

The fix instantiates ReceiveInterrupts in processConnection(), which
registers a callback to send SIGUSR1 to the current thread when
triggerInterrupt() is called. This allows the blocking read() to return
with EINTR, causing checkInterrupt() to throw and the daemon to exit
cleanly.

This pattern is already used in ThreadPool::doWork() and
SubstitutionGoal for the same purpose.

src/libstore/daemon.cc

@@ -1025,6 +1025,7 @@ void processConnection(ref<Store> store, FdSource && from, FdSink && to, Trusted
 #ifndef _WIN32 // TODO need graceful async exit support on Windows?
     auto monitor = !recursive ? std::make_unique<MonitorFdHup>(from.fd) : nullptr;
     (void) monitor; // suppress warning
+    ReceiveInterrupts receiveInterrupts;
 #endif
 
     /* Exchange the greeting. */