CI: build nixos manual also on PRs targeting secondary branches

[m1-s]

In #491742 we discovered that the nixos manual is not built on PRs targeting staging-nixos branch. This is because the nixos manual is only built on PRs that target primary branches and staging-nixos is a secondary branch. The nixpkgs manual in contrast is being built on secondary branches so it seems that this is an inconsistency. This PR fixes this by adjusting the nixos manual build conditions to match the ones from the nixpkgs manual.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

[Ma27]

cc @NixOS/nixpkgs-ci

Doesn't that mean we try to schedule manual builds for staging which will timeout anyways after consuming a whole lot of resources?

Perhaps we should explicitly allow staging-nixos here as well?
(Mostly a question to the CI team, not a proposal since I'm not sure I understand it correctly)

[vcunat]

It certainly seems an overkill to build the manual when targeting staging or staging-25.11.

[MattSturgeon]

Going of memory here, so I could be wrong. But I believe the manual is built using packages from the ci-pinned nixpkgs. Only the markdown sources come from the PR branch.

[m1-s]

I think thats true for the nixpkgs manual but not for the nixos manual.

manual-nixos = (import ../nixos/release.nix { }).manual.${system} or null;
manual-nixpkgs = (import ../doc { inherit pkgs; });

In the nixos/release.nix nixpkgs is declared as:

  nixpkgs ? {
    outPath = cleanSource ./..;
    revCount = 708350;
    shortRev = "gfedcba";
  },

So I assume the nixos manual uses the current state that is in the PR. Is rebuilding the nixos manual feasible in PRs to staging-nixos or does it cost too much resources? Do we only want to allow staging-nixos or all secondary branches?