[CI] Add CI workflow configuration

* CI for testing OTB builds in Nix.

.github/workflows/ci.yml

@@ -0,0 +1,19 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+        with:
+          diagnostic-endpoint: ""
+      - run: nix build .