RemoteStore::connectToDaemon(): Set close-on-exec flag

This ensures that "nix-build --run-env" doesn't keep a connection to
the worker open, preventing it from exiting.

src/libstore/remote-store.cc

@@ -141,6 +141,7 @@ void RemoteStore::connectToDaemon()
     fdSocket = socket(PF_UNIX, SOCK_STREAM, 0);
     if (fdSocket == -1)
         throw SysError("cannot create Unix domain socket");
+    closeOnExec(fdSocket);
 
     string socketPath = settings.nixStateDir + DEFAULT_SOCKET_PATH;