feat: Update ssg cache workflow

.forgejo/workflows/cache-ssg.yaml

@@ -5,6 +5,7 @@ on:
   push:
     paths:
       - 'ssg/**'
+      - '.forgejo/workflows/cache-ssg.yaml'
 
 jobs:
   build-cache:
@@ -15,7 +16,7 @@ jobs:
           path: dgnum.eu
 
       - name: Install sudo
-        run: apt update && apt install sudo
+        run: apt-get update && apt-get install sudo
 
       - name: Install Nix
         uses: https://github.com/cachix/install-nix-action@v22
@@ -24,8 +25,7 @@ jobs:
 
       - name: Install attic, build and cache
         run: |
-          nix-env -iA npins
+          nix-env -i npins -f '<nixpkgs>'
 
           npins init
           npins add github zhaofengli attic -b main
-