Get flake inputs from substituter


‘nix build’ seems to always download the flake inputs from GitHub rather than the substituter (but I might be missing something).
I expected that the narHash in the lock file would allow to download from the substituter.
Is it possible, or should I rather mirror nixpkgs to avoid downloading from GitHub in this case?

I indeed can observe the inputs being substituted. At least those that can be substituted.

I do not expect any input to be substitutable though, except for nixpkgs itself.

Hmmm I was using nix 2.8 but that’s probably too old, seems to work as expected with 2.16!