ci: add basic nix and cachix support (#15)

.github/workflows/nix.yml

@@ -0,0 +1,15 @@
+name: "Nix"
+on:
+  push:
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: cachix/install-nix-action@v8
+    - uses: cachix/cachix-action@v5
+      with:
+        name: nix-community
+        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+        # Only needed for private caches
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'