workflows/manual-nixpkgs: build nixpkgs manual on staging and stable branches

By using the pinned nixpkgs we have for CI, we can lift the restriction
of building the nixpkgs manual only in PRs targeting master.

At the same time, this uses the pinned nixpkgs for the doc/ folder's dev
shell. This allows entering that shell while working on a staging-based
branch and write documentation.

Why should staging be un(der)documented, after all?

Note: The package that is available in nixpkgs as pkgs.nixpkgs-manual
will still be built with the current nixpkgs checkout, not the pinned
version. This is the same that hydra builds.

0 files changed, 0 insertions, 0 deletions