feat: Update ssg cache workflow

.forgejo/workflows/cache-ssg.yaml

@@ -0,0 +1,31 @@
+name: Cache the SSG build
+run-name: cache-ssg
+
+on:
+  push:
+    paths:
+      - 'ssg/**'
+      - '.forgejo/workflows/cache-ssg.yaml'
+
+jobs:
+  build-cache:
+    runs-on: debian-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          path: dgnum.eu
+
+      - name: Install sudo
+        run: apt-get update && apt-get install sudo
+
+      - name: Install Nix
+        uses: https://github.com/cachix/install-nix-action@v22
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
+      - name: Install attic, build and cache
+        run: |
+          nix-env -i npins -f '<nixpkgs>'
+
+          npins init
+          npins add github zhaofengli attic -b main