ci: add cachix build and push

.github/workflows/cachix.yml

@@ -0,0 +1,15 @@
+name: "Cachix"
+on: 
+  pull_request:
+  push:
+jobs:
+  cachix-push:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v25
+    - uses: cachix/cachix-action@v14
+      with:
+        name: nixvim-superredstone
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - run: nix build -L .#default