Build a minimized Nix with MinGW

At this point many features are stripped out, but this works:

- Can run libnix{util,store,expr} unit tests
- Can run some Nix commands

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Brian McKenna <brian@brianmckenna.org>

src/libutil/thread-pool.cc

@@ -81,8 +81,10 @@ void ThreadPool::doWork(bool mainThread)
 {
     ReceiveInterrupts receiveInterrupts;
 
+#ifndef _WIN32 // Does Windows need anything similar for async exit handling?
     if (!mainThread)
         unix::interruptCheck = [&]() { return (bool) quit; };
+#endif
 
     bool didWork = false;
     std::exception_ptr exc;