nix: Adopt Determinate Nix as the common Nix

Use Determinate Nix as `nix.package` for systems, homes and the
devshell, for its parallel evaluation and lazy trees. We only take the
package, not `determinate-nixd`: the daemon and `nix.conf` model are
unchanged and the Determinate NixOS module is not imported.

- Add the `determinate-nix` (`nix-src`) input, following our
  `nixpkgs-unstable`. FlakeHub's cache needs auth, so we build it
  ourselves and let it flow through Harmonia like everything else.
- `determinateOverlay` exposes it as `pkgs'.mine.determinate-nix`;
  `lib.my.c.nix.determinateSettings` (`lazy-trees`, `eval-cores = 0`)
  is merged into `nix.settings` and the devshell `nix.conf`.
- Switch CI to `DeterminateSystems/determinate-nix-action` so the
  runner itself evaluates with Determinate.
- Advertise the Harmonia cache via the flake's `nixConfig`, trusted
  without a prompt via `accept-flake-config` in the devshell, `.envrc`
  and CI only (boxes already trust it through `nix.settings`).
- Re-attach `pkgs`/`lib` to container and installer
  `nixosConfigurations` so Determinate's flake schemas can evaluate
  them (`nix flake check` otherwise fails with `attribute 'pkgs'
  missing`).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/constants.nix

@@ -111,6 +111,13 @@ rec {
         extra-trusted-public-keys = ${concatStringsSep " " keys}
       '';
     };
+
+    # Determinate-specific settings enabling its performance features. Only understood by the
+    # Determinate Nix binary, so they must not be emitted for a base-Nix package.
+    determinateSettings = {
+      lazy-trees = true;
+      eval-cores = 0;
+    };
   };
 
   pubDomain = "nul.ie";