Added Nix CI

.github/workflows/nix.yml

@@ -0,0 +1,10 @@
+name: Nix CI
+
+on: [pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - uses: cachix/install-nix-action@v3
+      - run: nix-build

default.nix

@@ -0,0 +1,5 @@
+{ pkgs ? import <nixpkgs> {}
+}:
+pkgs.haskellPackages.developPackage {
+  root = ./.;
+}