Use proxy for nix build temporary

The connection to cache.nixos.org is sometimes very slow in my area. I would like to use a proxy but I just want to use it temporary when I need it. Is that possible for NixOS?

So fetchTarball running in the nix repl will respect the *_proxy variables but it seems not affect nix-instantiate and nix-build command, why is that?

Where are you all from?