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/libexpr/flake/flake.cc

@@ -307,7 +307,7 @@ LockedFlake lockFlake(
 
     if (lockFlags.applyNixConfig) {
         flake.config.apply();
-        // FIXME: send new config to the daemon.
+        state.store->setOptions();
     }
 
     try {

src/libstore/remote-store.cc

@@ -290,6 +290,10 @@ ConnectionHandle RemoteStore::getConnection()
     return ConnectionHandle(connections->get());
 }
 
+void RemoteStore::setOptions()
+{
+    setOptions(*(getConnection().handle));
+}
 
 bool RemoteStore::isValidPathUncached(const StorePath & path)
 {

src/libstore/remote-store.hh

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

src/libstore/store-api.hh

@@ -724,6 +724,11 @@ public:
     virtual void createUser(const std::string & userName, uid_t userId)
     { }
 
+    /*
+     * Synchronises the options of the client with those of the daemon
+     * (a no-op when there’s no daemon)
+     */
+    virtual void setOptions() { }
 protected:
 
     Stats stats;