[bug#45018] [PATCH 3/6] daemon: Factorize substituter agent spawning.

[Mathieu Othacehe]

Hey Ludo,

> +     const std::map<string, string> env = { { "_NIX_OPTIONS", 
> settings.pack() } };
> +     runningSubstituter = std::make_shared<Agent>(settings.guixProgram, 
> args, env);

The only owner of this Agent is LocalStore, so why not using
make_unique? This function could return a const reference to the Agent
instead.

Thanks,

Mathieu