feat(workflows): Switch to nix runner

.forgejo/workflows/cache-ssg.yaml

@@ -1,31 +0,0 @@
-name: Cache the SSG build
-run-name: cache-ssg
-
-on:
-  push:
-    paths:
-      - 'ssg/**'
-
-jobs:
-  build-cache:
-    runs-on: debian-latest
-    steps:
-      - uses: actions/checkout@v3
-        with:
-          path: dgnum.eu
-
-      - name: Install sudo
-        run: apt update && apt 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 -iA npins
-
-          npins init
-          npins add github zhaofengli attic -b main
-

.forgejo/workflows/deploy-dgnum.yaml

@@ -8,19 +8,12 @@ on:
 
 jobs:
   publish:
-    runs-on: debian-latest
+    runs-on: nix
     steps:
       - uses: actions/checkout@v3
         with:
           path: dgnum.eu
 
-      - name: install sudo
-        run: apt update && apt install sudo
-
-      - uses: https://github.com/cachix/install-nix-action@v22
-        with:
-          nix_path: nixpkgs=channel:nixos-unstable
-
       - name: Build website
         run: |
           # Hack to set the cache in the working directory