mod: added nix install action

.gitea/workflows/publish-attic.yaml

@@ -8,18 +8,21 @@ on:
 
 jobs:
   publish:
-    runs-on: nixos/nix
+    runs-on: ubuntu-latest
 
     env:
       ATTIC_SERVER: ${{ secrets.ATTIC_SERVER }}
       ATTIC_CACHE: ${{ secrets.ATTIC_CACHE }}
       ATTIC_TOKEN: ${{ secrets.ATTIC_TOKEN }}
-      NIX_CONFIG: experimental-features = nix-command flakes
+      # NIX_CONFIG: experimental-features = nix-command flakes
 
     steps:
       - name: Check out repository
         uses: actions/checkout@v4
 
+      - name: Install upstream Nix
+        uses: cachix/install-nix-action@v31
+
       - name: Install Attic client
         shell: bash
         run: |