Make the flake options work when using the daemon

When setting flake-local options (with the `nixConfig` field), forward
these options to the daemon in case we’re using one.

This is necessary in particular for options like `binary-caches` or
`post-build-hook` to make sense.

Fix <343239fc8a (r44356843)>

src/libstore/remote-store.hh

@@ -147,6 +147,8 @@ protected:
 
     virtual void setOptions(Connection & conn);
 
+    void setOptions() override;
+
     ConnectionHandle getConnection();
 
     friend struct ConnectionHandle;