add sternchen to update process

.forgejo/workflows/nix_build.yaml

@@ -61,6 +61,9 @@ jobs:
       - name: nix build chungus
         run: nix build .#nixosConfigurations.chungus.config.system.build.toplevel
 
+      - name: nix build sternchen
+        run: nix build .#nixosConfigurations.sternchen.config.system.build.toplevel
+
       - name: commit & push
         if: ${{ github.event_name == 'schedule' }}
         # only if all nix builds are fine we update our branch