Use cachix to publish binary cache

.github/workflows/cachix.yml

@@ -0,0 +1,17 @@
+name: "Build and push to cachix"
+on:
+  pull_request:
+  push:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2.3.4
+    - uses: cachix/install-nix-action@v12
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: cachix/cachix-action@v8
+      with:
+        name: davegallant
+        authToken: '${{ secrets.CACHIX_API_KEY }}'
+    - run: nix-build | cachix push davegallant