attic for attic - (add substituter to nixConfig in flake.nix)

[tennox]

I want to use attic in many places (e.g. servers, CI) - but building it bothers me: it takes 20 minutes on some machines 😲

Now shouldn't attic have an attic cache?
I'm guessing from reading the GH Action logs that you might be using one, but it would be great if one were configured as substituter in flake.nix, as in this PR (example - it's pointing to my attic).

Now, this takes ~20s (weirdly still needs to build stuff, I guess for hash calc, but a lot less):

docker run nixpkgs/nix-flakes \
    nix build -L --verbose github:tennox/attic/attic-for-attic

(In docker to test with a clean store & config)

What you you think?

[tennox]

I'm guessing from reading the GH Action logs

I just read a bit more and found there is indeed an attic & with build push:
https://staging.attic.rs/attic-ci

So I'm guessing that is the one to use?
If so, you can merge attic-for-attic-staging branch.