add builder workflow

.github/workflows/nix.yml

@@ -0,0 +1,28 @@
+name: Nix
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+jobs:
+  build:
+    strategy:
+      matrix:
+        os:
+          - ubuntu-20.04
+        target-system:
+          - ymir
+          - tartarus
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v16
+      # TODO: add a binary cache
+      # - uses: cachix/cachix-action@v10
+      #   with:
+      #     name: YOURCACHE
+      #     authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+      - run: nix flake check
+        # Pre-build the system configuration
+      - run: nix build .#nixosConfigurations.${{ matrix.target-system }}.config.system.build.toplevel
+