flake: add cache config

[edrex]

This enables nix tooling to offer to enable the cache:

edrex@chip ~/s/g/B/nix-script (main)> nix profile install .
do you want to allow configuration setting 'extra-substituters' to be set to 'https://nix-script.cachix.org' (y/N)? y
do you want to permanently mark this value as trusted (y/N)? y
do you want to allow configuration setting 'extra-trusted-public-keys' to be set to 'nix-script.cachix.org-1:czo3tF6XERpkDdMd6m84XjmgeHQXNeIooSt7b0560+c=' (y/N)? y
do you want to permanently mark this value as trusted (y/N)? y

(doesn't work for inputs, only the root flake)

I didn't get a cache hit when I just tried it though.

[BrianHicks]

I feel a little confused: is this working and ready for review or not? You say you didn't get a cache hit. Do you know why?

[edrex]

That means your build is broken.

(Edit: this can be caused by not using the flake.lock nixpkgs version in your build, various nix config options)

[BrianHicks]

Can you say more?

[edrex]

Sorry, edit above

[edrex]

I saw this happen once before when https://github.com/srid/emanote added it, i think it was wrong nixpkgs version.

[BrianHicks]

Do you mean a differing nixpkgs between your installation and the one in this repo? Or something else?

[edrex]

flake run uses the version from the lock, unless i used --override-input nix-script github:my/fork. It must be the derivation output of flake.nix:packages.default (with inputs as flake.lock, including nixpkgs) isn't being uploaded to the cache.

[edrex]

I can take a look when I'm back at my laptop

[edrex]

Oops that should be something like --override-input nixpkgs nixpkgs (The second argument is evaluated against the local registry, so it gives you whatever you have pinned locally). That may be what is happening on the build.