fix(#1199): add nix to hive-ci PATH for runner job execution

The gitea-actions-runner's host-scheme job processes use the
service's environment PATH, not the NixOS login-shell PATH.
Without this, 'nix flake check' and similar steps fail with
'nix: command not found'.

- add pkgs.nix to environment.systemPackages (ensures the binary
  is in /run/current-system/sw/bin)
- set explicit PATH on gitea-runner-hive service covering both
  /run/current-system/sw/bin and /nix/var/nix/profiles/default/bin

nix/modules/hive-ci.nix

@@ -268,10 +268,22 @@ in
           systemd.services."gitea-runner-hive".wants = [ "hive-ci-register.service" ];
 
           # git is required by the runner's checkout step.
-          # Everything else (nix, rust tools) is either part of NixOS
-          # by default or pulled in hermetically by nix flake check.
+          # nix is required for `nix flake check` / `nix build` workflow
+          # steps — it's not in the runner's job PATH by default since
+          # systemd services don't inherit the NixOS login-shell PATH.
           # curl/jq in the register script use absolute store paths.
-          environment.systemPackages = [ pkgs.git ];
+          environment.systemPackages = [
+            pkgs.git
+            pkgs.nix
+          ];
+
+          # Expose /run/current-system/sw/bin (where system packages
+          # land) and the nix profile bin dir to the runner's job
+          # execution environment. The gitea-actions-runner host scheme
+          # spawns job processes with the service's environment; without
+          # this PATH override, `nix` and `git` aren't visible to steps.
+          systemd.services."gitea-runner-hive".environment.PATH =
+            "/run/current-system/sw/bin:/nix/var/nix/profiles/default/bin:/usr/bin:/bin";
         };
     };
   };