chore: remove attic

.github/workflows/flake.yml

···
       - name: Install nix
         uses: DeterminateSystems/nix-installer-action@main
 
-      - name: Setup Attic cache
-        uses: ryanccn/attic-action@v0
-        with:
-          endpoint: ${{ secrets.ATTIC_ENDPOINT }}
-          cache: ${{ secrets.ATTIC_CACHE }}
-          token: ${{ secrets.ATTIC_TOKEN }}
-
       - name: Build
         run: nix develop -c bash -c "set -euo pipefail; just build"
 

flake.nix

···
         };
       };
     };
-  nixConfig = {
-    extra-substituters = ["https://attic-testing.fly.dev/system"];
-    extra-trusted-public-keys = ["system:jO6HbDP3xnQbHy/llnSubs8NPK7cVWwG827k6inRZkY="];
-  };
 }