Send the right nix-serve client version

We were using protocol version 6 but requesting version 4. The only
reason that this worked was because of a broken version check in
'nix-store --serve'. That was fixed in
https://github.com/NixOS/nix/commit/c2d74569269d9642448a701aa6efe08ec0fe6f00,
which had the side-effect of breaking hydra-queue-runner.

src/hydra-queue-runner/build-remote.cc

@@ -234,7 +234,7 @@ void State::buildRemote(ref<Store> destStore,
         unsigned int remoteVersion;
 
         try {
-            to << SERVE_MAGIC_1 << 0x204;
+            to << SERVE_MAGIC_1 << 0x206;
             to.flush();
 
             unsigned int magic = readInt(from);