ci: add github-actions

.github/workflows/nix.yml

@@ -0,0 +1,23 @@
+name: Nix
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+  workflow_dispatch:
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ ubuntu-20.04 ]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v12
+      - uses: cachix/cachix-action@v8
+        with:
+          name: nix-community
+          signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+      - run: ./build
+        env:
+          - NIX_PATH=channel:nixos-unstable