ci: nix based image

.forgejo/workflows/check.yml

@@ -3,20 +3,15 @@ on: [push]
 
 jobs:
   Check:
-    runs-on: ubuntu-latest
-    env:
-      USER: root
+    runs-on: nix-flakes
     steps:
       - name: Check out repository code
         uses: https://code.forgejo.org/actions/checkout@v4
-      - name: Install sudo
-        run: "apt update && apt install -y sudo"
-      - name: Install nix
-        uses: https://github.com/cachix/install-nix-action@v23
       - name: Prepare cachix
         uses: https://github.com/cachix/cachix-action@v12
         with:
           name: pub-solar
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
       - name: Run flake checks
         run: |
           nix --print-build-logs --verbose --accept-flake-config flake check