chore: add checking nix flake in ci

Author: itscrystalline

.github/workflows/ci.yaml

@@ -51,6 +51,25 @@ jobs:
         run: devenv tasks run occasion:test
       - name: Generate coverage report
         run: devenv tasks run occasion:coverage
+      - name: Try to build with nix
+        run: devenv shell nix build .#occasion-latest
+
+      - name: Upload coverage report 
+        uses: coverallsapp/github-action@v2
+
+  nix:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v26
+      - uses: cachix/cachix-action@v14
+        with:
+          name: devenv
+      - name: Install devenv.sh
+        run: nix profile install nixpkgs#devenv
+
+      - name: Try to build with nix
+        run: devenv shell nix build .#occasion-latest
 
       - name: Upload coverage report 
         uses: coverallsapp/github-action@v2