GC server: Clear O_NONBLOCK on the right file descriptor

The bug fix in 6d30f9e6fe erroneously
cleared O_NONBLOCK on the server rather than client FD (leaving both
in an incorrect state).

Fixes #8551.

(cherry picked from commit a6a75ecad8)

src/libstore/gc.cc

@@ -564,7 +564,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
                     /* On macOS, accepted sockets inherit the
                        non-blocking flag from the server socket, so
                        explicitly make it blocking. */
-                    if (fcntl(fdServer.get(), F_SETFL, fcntl(fdServer.get(), F_GETFL) & ~O_NONBLOCK) == -1)
+                    if (fcntl(fdClient.get(), F_SETFL, fcntl(fdClient.get(), F_GETFL) & ~O_NONBLOCK) == -1)
                         abort();
 
                     while (true) {