Cache miss on self-hosted CI

Help
1

I have a self-hosted nix-serve-ng on my CI (Github Actions, which runs a nix build on each push).

On another hand, I have a flake.nix which starts with:

{
  nixConfig.extra-trusted-substituters = "https://nix.acme";
  nixConfig.extra-trusted-public-keys = "nix.acme:/kJVKDt...rc=";

...
    defaultPackage = haskelPackages.callCabal2nix ...

However, even though the CI run, I have to rebuild it locally on each nix build.

I have tried

nix-eval-jobs --check-cache-status --flake '.#defaultPackage' | jq -r '.'
warning: unknown setting 'allowed-users'                                       
warning: unknown setting 'trusted-users'                                       
warning: `--gc-roots-dir' not specified                                                                                                                       
Using saved setting for 'extra-trusted-public-keys = nix.acme:/kJVKDt...rc=' from ~/.local/share/nix/trusted-settings.json.
Using saved setting for 'extra-trusted-substituters = https://nix.acme' from ~/.local/share/nix/trusted-settings.json.
{                                                                              
  "attr": "x86_64-linux",                                                      
  "attrPath": [                                                                
    "x86_64-linux"                                                             
  ],                                                                           
  "drvPath": "/nix/store/wii...i-myproject-1.2.3.drv",
...
  },
  "isCached": false,
  "name": "myproject-1.2.3",
  "outputs": {
    "out": "/nix/store/i4i6qlh7r4...-myproject-1.2.3"
  },
  "system": "x86_64-linux"
}

Both paths exists on my CI (and nix build is mostly a no-op on it).

Is there anything I have to configure to have a cache it?

2

There may be an issue with trusted users if it’s using nix daemon service and that your user in the CI is not trusted, the flake can’t use the substituter.

3

I added this line in my /etc/nix/nix.conf:

trusted-users = @sudo github-runner

but it has no effect

I don’t know if it can have an impact, but nix-serve-ng is running as github-runner too

4

I have actually tried to run nix-serve-ng --verbose, it seems it is never hit.