Add nix flake to repo (#1111)

.github/workflows/nix-build.yml

@@ -0,0 +1,18 @@
+name: Nix build
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build-nix:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v25
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: DeterminateSystems/magic-nix-cache-action@v2
+    - run: nix build -L
+    - run: nix develop --command echo OK