Add a flake with a NixOS module to generate files

[nlewo]

We previously used a buildkite agent to run scripts. However, this agent has been removed by nix-community admins.

Instead of running a buildkite agent and using GitHub to serve produced artefacts, it seems we could run scripts thanks to a systemd periodic task and serve artefacts with Nginx: nix-community/infra#445

In this PR, a flake exposes the nixpkgs-swh-generate program and a NixOS module running this program each days.

I'm currently testing this PR on my server (it only generates the list of sources for the hello attribute): http://nixpkgs-swh.abesis.fr

The URL used by the Software fetcher will have to be updated (it is currently https://nix-community.github.io/nixpkgs-swh/sources-unstable-full.json).

[nlewo]

@ardumont We will have to update to URL used by the SWH nixguix component (because i'm planning to leave GitHub Pages). I'm not able to find it in your GitLab instance and i don't remember where it is configured ;)

Would you be ok to update this URL (once i provided it) on the SHW side?

[ardumont]

@nlewo hello ;)

@ardumont We will have to update to URL used by the SWH nixguix component (because i'm planning to leave GitHub Pages).

ack

I'm not able to find it in your GitLab instance and i don't remember where it is configured ;)

That one is not declared in the code, it's in the scheduler db iirc.

Would you be ok to update this URL (once i provided it) on the SHW side?

Sure thing, i'll update it when it's ready.

Thanks in advance.

[ardumont]

(and cool for the flake work ;)

[zowoq]

I pushed a couple of minor fixes, I think they should be self explanatory.

[nlewo]

Closing because integrated in #10