ci-nix github action

* Fixes #9, missing build with nix in CI

* Once the project grows, we should select better, or a default, build
  targets

.github/workflows/ci-nix.yaml

@@ -0,0 +1,32 @@
+# https://github.com/marketplace/actions/install-nix
+
+name: "ci-nix"
+on:
+  pull_request:
+  push:
+
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+
+    - uses: actions/checkout@v4
+
+    - name: Install nix
+      uses: cachix/install-nix-action@v27
+
+    # Possibly need to cache cabal.
+    - name: Cache .cabal
+      uses: actions/cache@v3
+      with:
+        path: ${{ steps.cabal-setup.outputs.cabal-store }}
+        key: cabal-${{ hashFiles('cabal.project') }}
+
+    # We currently do not have a default target
+    - run: nix build .#wst-poc
+    - run: nix flake check
+