add demo test workfloz

.github/workflows/demo.yml

@@ -0,0 +1,16 @@
+name: "Demo"
+on:
+  pull_request:
+  push:
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v22
+      with:
+        github_access_token: ${{ secrets.GITHUB_TOKEN }}
+    - run: |
+      for d in ./demo;
+        do nix flake check
+      done