Use LegacySSHStore

In https://github.com/NixOS/nix/pull/10748 it is extended with
everything we need.

src/hydra-queue-runner/state.hh

@@ -20,9 +20,7 @@
 #include "store-api.hh"
 #include "sync.hh"
 #include "nar-extractor.hh"
-#include "serve-protocol.hh"
-#include "serve-protocol-impl.hh"
-#include "serve-protocol-connection.hh"
+#include "legacy-ssh-store.hh"
 #include "machines.hh"
 
 
@@ -292,9 +290,11 @@ struct Machine : nix::Machine
     bool isLocalhost() const;
 
     // A connection to a machine
-    struct Connection : nix::ServeProto::BasicClientConnection {
+    struct Connection {
         // Backpointer to the machine
         ptr machine;
+        // Opened store
+        nix::ref<nix::LegacySSHStore> store;
     };
 };