ci: add cachix tests

.github/workflows/main.yml

@@ -0,0 +1,18 @@
+name: "Test"
+on:
+  pull_request:
+  push:
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v25
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: cachix/cachix-action@v14
+      with:
+        name: suntheme
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - run: nix build
+    - run: nix develop