build-remote: Fix fallback to other machines when connecting fails

Opening an SSHStore or LegacySSHStore does not actually establish a
connection, so the try/catch block here did nothing. Added a
Store::connect() method to test whether a connection can be
established.

src/libstore/remote-store.hh

@@ -92,6 +92,8 @@ public:
         PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown,
         unsigned long long & downloadSize, unsigned long long & narSize) override;
 
+    void connect() override;
+
 protected:
 
     struct Connection