Add flake-based GitHub Actions workflow

.github/workflows/test-flakes.yml

@@ -0,0 +1,31 @@
+name: "Flake test"
+on:
+  pull_request:
+  schedule:
+    - cron:  '51 2 * * *'
+jobs:
+  tests:
+    strategy:
+      matrix:
+        os: [ ubuntu-latest, macos-latest ]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - uses: actions/checkout@v2
+      with:
+          # Nix Flakes doesn't work on shallow clones
+          fetch-depth: 0
+    - uses: cachix/install-nix-action@v12
+      with:
+        install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20201221_9fab14a/install
+        extra_nix_config: |
+          experimental-features = nix-command flakes
+          system-features = nixos-test benchmark big-parallel kvm
+    - name: Setup cachix
+      uses: cachix/cachix-action@v8
+      with:
+        name: mic92
+        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+    - name: List flake structure
+      run: nix flake show
+    - name: Run unit tests (flake)
+      run: nix build --no-link .#unit-tests -L