ci: Add workflow that runs nix build

.github/workflows/build-flake.yml

@@ -0,0 +1,19 @@
+name: Build Nix Flake
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  build-flake:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - uses: cachix/install-nix-action@v25
+
+      - run: nix build