mod: added sandbox to nix config

.gitea/workflows/publish-attic.yaml

@@ -22,6 +22,10 @@ jobs:
 
       - name: Install upstream Nix
         uses: cachix/install-nix-action@v31
+        with:
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+            sandbox = true
 
       - name: Install Attic client
         shell: bash
@@ -32,7 +36,7 @@ jobs:
             exit 0
           fi
 
-          nix profile install --accept-flake-config nixpkgs#attic-client
+          nix profile add --accept-flake-config nixpkgs#attic-client
           echo "$HOME/.nix-profile/bin" >> "$GITHUB_PATH"
 
       - name: Configure Attic