ci: add cachix build cache

.github/workflows/cachix.yml

@@ -0,0 +1,20 @@
+name: "Build and push binaries to cachix"
+on:
+  push:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Install nix
+        uses: cachix/install-nix-action@v30
+      - name: Setup cachix action
+        uses: cachix/cachix-action@v14
+        with:
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+          name: polkadot
+      - name: Nix flake check
+        run: nix flake check
+      - name: Nix flake develop
+        run: nix develop